It's the standard means for them to be able to offer the gtd that they do...without the reentry the gtd would be smaller and in turn get less people playing them ... rince repeat and you will get gtd prizepools dropping which isn't what players want.
WPN mtt's are the same for the most part, FTP have many of their games multi-entry (or recently multi-day, where you can have even multiple entries on multiple days to try and qualify to day2) which means you can run multiple seats in the same game or use them as reentry. It's almost the only way that sites without a lot of traffic can offer gtd prizepools that will entice more people to play them.
Regarding carbon they have recently gone on a progressive gtd kick which means that each week that the gtd prizepool is met or exceeded in specific tournies they will increase the gtd prizepool the next week.........so basically bigger prize pool attracks more players and more players with this progressive gtd approach makes bigger prizepools and so on.
I see reasons for not liking the reentry events, personally I would prefer a rebuy to a reentry and i'm not a fan of rebuy's either but at least in a rebuy you can rebuy a few times and expect that a min cash will at least cover your entry...in a reentry however since each entry is counted as a new player once you enter a couple of times you need to run deep even just to break even.......obv we are playing for FT and top3 but just as basis to where typically you expect at least an ITM is entry back, well it's not the case if you enter more than once.
Typically just play it no different than any other game, buyin one and if you bust thats it don't reenter but the reentry does serve it's purpose of boosting prizepools or at least producing low overlay so the prizepools won't drop anytime soon.