When George launches into his tirade about what he could have done had he not been saddled with the responsibility of Lennie, it reveals a couple of large scale implications regarding the book and its characterizations. The first is that the speech shows how much of a struggle solidarity is in a time of economic despair. When resources are so scarce, to uphold the bonds of solidarity are extremely difficult. I think that this is something that is brought out in the speech. George balances the demands of financial scarcity with social solidarity. In this, he achieves an almost heroic quality.
Another element brought out in the speech is how truly alone both of them are. An unintended effect of George's speech is one that shows how Lennie is nothing without George. George is the brains behind the operation, and like a classical hero, "does the thinking for both of them." I think that this is brought out in all that George could do without Lennie. Yet, at the same time, George brings out that he, himself, is alone. In bringing out the vision of the farm that is shared between them at the end of his tirade, George subconsciously admits that he is just as alone as Lennie is. Both of their senses of "aloneness" end up defining them and helps to explain why they are together.
I think that the final implication of the speech is that George understands that his being in the world is defined as Lennie's caretaker. We understand later why this is the case. Yet, George reveals that his primary purpose of taking care of Lennie and protecting him is a part of his character. It also serves to foreshadow how their relationship will emerge out over the course of the novel. Steinbeck's genius in bringing out this tirade early helps to establish the arc of their relationship over the course of the novella.