BOSTON — This is why the home court counts.

This is why all those games in January are important, even though the rest of the world is watching the Patriots.

This is why all the games from the West Coast in February are important, all those games that no one ever watches because they’re on television too late.

This why all these games in this NBA season that seems to go on forever are important, all those games that seem to run together, for the simple reason that there are simply too many of them, game after game, 82 of them.

This is why the regular season matters.

For that’s what gives the Celtics the home-court advantage in this series with the young, upstart Hawks, the reason the Celtics will have the home-court advantage in every playoff series they play. The reason why, once again, the Celtics are back in control of this series.

The only thing that counts in the NBA is the playoffs?

No doubt.

But this is how you win series.

Last night was Exhibit A.

Game Five, the traditional deciding game of an NBA series.

Here in The Garden.

Not in Atlanta where the crowd is alive and festive and the Hawks feed off their energy and make shots that they don’t make on the road. But here in The Garden, where the Hawks were blown out in the first two games, looking more like robins in search of a nest than a team that was going to upset the Celtics. Here in The Garden, where the Celtics returned last night in search of the magic that saw them win 66 games in the regular season, the magic that’s made them the favorite to get to the NBA Finals.

Thank the basketball gods.

Truth be told, the Celtics didn’t look like a team that had won 66 games in the two games in Atlanta, didn’t look like a team that’s supposed to bring back the glory days. For the first time they looked vulnerable, both their lack of athleticism and their inability to make key defensive stops. The Hawks were the ones that brought the passion and energy. The Hawks were the team that set the tone.

The Celtics?

The Celtics looked like we hadn’t seen them look all year, just another team getting beat on the road.

For make no mistake: this series was not supposed to be tied, 2-2, going into last night. Not against a team that wasn’t even .500 in the regular season, a team fortunate to even be in the playoffs, never mind thinking they could actually win a series.

So last night was what we expected when this series started, the Celtics taking control of the game early, cruising to a 15-point lead at the half. In many ways it was déjà vu all over again, as Yogi used to say, a game very reminiscent of the first two.

Once again, the Celtics came out and took control of the game early, both their defensive intensity and their good shot selection getting them out in front early, fueled by the crowd and the incredible din that’s become a home game in The Garden. Once again, as they did in the first two games, the Hawks seem to wilt right in front of our eyes, unable to make shots, unable to match the Celtics’ intensity.

Throw in Sam Cassell’s nine points in the second quarter, and the off-the-bench intensity of Leon Powe and the first half was a page out of a Doc Rivers game plan.

This was the same team that had played the two games in Atlanta?

Didn’t look like it.

Instead, it looked like the Celtics we’ve come to know all year, a balanced team that knows how to win.

And the Hawks?

They, too, looked like the team we expected when this series started — young, inexperienced, not yet ready to win a game of this magnitude on the road. In short, last night was a microcosm of how this series was supposed to play out.

What’s the old adage? A series doesn’t really begin until the home team loses a game?

Well here we are five games into this series and the Celtics still haven’t won one.

No matter.

They are up, 3-2, one game away from ending this series and sending the Hawks home for the summer, even if this series is already going to last longer than anyone thought it would when it started. And they are up, 3-2, for the simple reason that they have now won three home games.

And, yeah, the Hawks didn’t play nearly as well as they had in Atlanta, even if they hung in longer than they did in the first two games in Boston. No surprise there. Learning to win on the road is last thing young teams learn to do, the last piece of the puzzle.

Why is it so hard?

“Because they’re young,” Atlanta coach Mike Woodson said ruefully.

“It’s like we’re two different teams at home and one the road,” said the Hawks’ Joe Johnson.

No mystery there, either.

Last night might have been different if it had been in Atlanta.

It wasn’t.

It was here in The Garden, this place where the Celtics have seldom lost this year, this place where the storied history of this franchise has become the sixth man, all those old ghosts of the past helping push this team into the future.

One more reminder of how much the home court means in these NBA playoffs.

One more reminder that all those games in January that no one really cares about really do matter.

One more reminder of the importance of the home-court advantage.

The reason the Celtics now lead, 3-2, in this series.

breynold@projo.com