They will often discuss how to attack the next hitters (maybe one is weak on outside sliders, for example) and remind the pitcher of the situation. In particular the discussion will sometimes hinge around whether the pitcher/catcher/coach want to pitch carefully to the next hitter (and likely walk him) in order to face the following hitter.
Also, coaches are looking for signs from the pitcher that will tip off their fatigue and confidence levels.
Sometimes it is just small talk… especially early in a game… If a pitcher is rattled a coach might come out to talk to him just to give him a chance to refocus himself and get back on track…
Late in the game it is also used as a stall tactic while another pitcher warms up in the bullpen…
a rule that not everyone is aware of is that coaching staff only has one “free” visit to the mound per inning per pitcher… If a manager comes out a second time the pitcher has to be pulled from the game…
How, specifically, to pitch the next batter, esp. if the manager elects to pitch to him in a way that differs from the pre game plan.
How to arrange the defense.
Observations that the manager or pitching coach have concerning the pitcher’s mechanics. They will often see things that the pitcher doesn’t know and catcher doesn’t notice.
Frank Crocetti: “C’mon, Lefty, bear down, the bases are loaded!”
Lefty Gomez: “I know they’re loaded. Did you think I thought they gave me an extra infield?”
Tommy Lasorda’s infamous mound visit in the 1977 World Series (audio NSFW). As one of the comments says, the organ music in the background makes it much funnier.
As well as this rule, there’s also a rule which states that only three “free” mound visits are allowed per pitcher for the duration of the game; if a fourth visit is made to the same pitcher, he must be removed.
That was such a great game. I was watching it and as soon as it happened, I turned to Suburban Plankton and said, “He stepped off, he stepped off!!!” and sure enough, Bochy walks out to argue it.