Thats a bit of reductionist view, xp gates started of as a fun mechanic. You play the base game, and unlock new weapons that you try out and compare with older

Thats a bit of reductionist view, xp gates started of as a fun mechanic. You play the base game, and unlock new weapons that you try out and compare with older ones. You get sense of progression and something new to play with. Its a good mechanic to keep player interested and expose them to different play-style. Problems starts when money-people starts to poke around with this and design it for exploitation, making XP levels too big, making starting weapons worse and lategame weapons overpowered. After that they give you solution of buying XP booster etc. They literally break their game in order to sell you solution.