Goalie is one of those positions, where there is only so much ice time between the NHL and AHL squads (4 spots, basically). As such, it isn't bad to simply go out and trade for a guy who is being stuck in a numbers game at the age of 20 (no room at the AHL level) or being stuck in a contract waivers issue later (like we got Bishop from the Blues).
Looking at a team like Tampa again has Gudlevskis as their back-up in the AHL, might make sense to trade for him to be our Bingo Starter. Or a team like Anaheim might trade Bobkov, because they simply don't have room to let him play with Gibson ahead of him as the heir apparent. There are other teams in similar situations, but the advantage is you get to trade for a guy with less risk given you have seen them succeed at the higher levels when drafted, but typically don't have to give up a higher pick than when drafted. Even drafting goalies in the 2nd round, half seem to bust within 3-4 years. 1st rounders aren't much better.