Made of some material (commonly heat resistant mesh which a wire frame holds rigid) a brass trap or catcher also typically mounts to a section of Pic rail along your MSR’s handguard and captures spent cases, preventing them from flying all over your shooting area.
Without a doubt - but that doesn’t mean you shouldn’t add a brass catcher to your inventory.1.
As a courtesy to neighbors at the rangeHonestly, just the fact that brass catchers capture the ejected brass and prevent hot, spent brass from being scattered all about your shooting area is a good reason to get one because it’s quite a courtesy to your neighbors at the range.Everything to the right of your ejection port will be subject to a hail of fire otherwise; it’s just polite to use brass catchers in the company of other shooters when your own platform is an autoloader.2.
Either collect it and wait for a sunny day or reload now if you have the resources to do so.3.
Because you’re an environmentalist at heartFor those of you that hunt with your MSRs, the responsible thing to do is not to leave your spent brass in the bush.
To comply with range rulesThere is also the highly pragmatic observation of the fact that some ranges and competitions flatly require the use of some brass catcher or device to corral your spent cases.