As you might imagine, you can hit this ceiling in very short order, which is good enough reason on its own to enable it. TOTAL memory, as in video + system memory (which are two separate things, if you don't know). If you don't enable this flag, the game can never address more than 2GBs of memory. Why this flag exists and is needed is something that would take a lot of time and effort to explain, so instead I will direct you to this link (but you don't need to know this stuff to use it).
For those not in the know, Large Address Aware is an executable flag that tells Windows that a program can handle more than 2GBs of memory.