Well some do because they are rake-racing or points racing and its a way for them with a minimum bankroll to reduce the swings while playing higher limits.
It´s also usually a good indication that they are not at all comfortable with the flop-turn-river play, and want to get their money in when they think they might be a 2-5% favorite every time (ie. QQ vs AK) and take advantage of the other guys raising to try to get as much money in the pot as possible and shift those "coinflips" all the more in the favor of pot odds for the long run.
I dont hate these guys, of course I just like most other players that "make money" (or used to, since I am "Ex" player at this time), because I dont find them to do much harm and they are actually overall pretty weak.
But honestly, it sucks to sit on a table, get that one hand against one of these guys (a coinflip or maybe you dominated them and they pulled a win out of thier asses) and then they leave. That just isnt what poker is about, that is gambling (for my encounter that is).
It also hurts legitimate low-rollers (which I think is fine) who will take their 100 USD and buy-in for $20 at a time instead of $100 at a time and make 1 buy-in into 5x buy-ins effectively...
By the way, I wont name names, but some training sites do this because they earn % on the players, and the higher they play, the higher they rake while playing so they are not always teaching it purely to make sense for the players, but with an agenda of their own. Please always be a bit wary of things like that :)
