Why is a player at defense sometime throw the baseball to the first base,...

[MattH1]

...even if he catch it in the air? Baseball rule question

 

[JoeFinkle]

Dawgdays answer is correct in terms of the rules. Also, sometimes just as a custom, players will throw the ball around the infield after an out, especially if nobody is on base, and more often throwing to first than anywhere else. There is nothing in the rules about that, it's just a custom.

 

[dawgdays]

A runner must retouch his original base after a fly ball is caught. If he, or the base, is tagged before he retouches, he's out.

So, if there was already a runner at first base, the fielder catches the ball and throws it to first so the fielder there can tag the base or the runner before the runner makes it back.