for one, you could gain new perspectives on certain issues you may never have been inclined to consider in the first place; for two, you could gain insight on something totally new or outside of your own arena which would serve to keep you updated on the outside world or help you store up for conversation topics which would make you appear well-read and knowledgeable in front of others (notwithstanding that you might only know enough to pass off as that).
there, you can see the benefit of one man not remaining, so called, "an island". you remain the subject matter expert on one thing. your partner/best friend/sibling/whatever does his/her own research on his/her subject matter and then proceeds to summarise, condense and translate it for you in the most layman way that more or less allows you to make good sense of it. and vice versa.
both gain in this exchange, in the sense that, as the "recipient", you learn something new you might not have learnt otherwise or might have spent a lot more time finding out about it given your lack of expertise on the matter; and as the "giver", you practise articulating your points, you discuss, explore and argue about it to better make sense of it, dumping the unimportant parts, storing the crucial, amending flawed logic, patching loopholes, where discovered... until finally perfected, you'd be more than ready to unleash the topic on the outside world who would likely be judging you by your knowledge of it.
it isn't that there must always be such practical advantages to any kind of relationship, it's just that, in some way or other, this is what relationships are about anyway, aren't they - that each stands to gain somewhere, derives something out of it, that allows the relationship to continue moving forward. in a way, this is companionship, no? if one has nothing to give, or one has no desire to receive, then it kind of seems to me that that one has totally no need to have any kind of relationship with the other anymore, as that one would then already be totally independent of the other anyway. do i make sense?
what philosophical crap. but i write, (1) in the vain attempt to keep my writing skills in shape ('tho i could do with much shorter sentences in "real" writing), (2) to work out my messy thoughts in somewhat nice, neat, logically-linked paragraphs (as far as possible), which would consequently (3) allow me to articulate myself in a much more well-thought-through (and hence hopefully more persuasive, more convincing) manner if ever need be.
oh, and (4), so that the world (i.e. whoever chances by) may know me for my thoughts, because whatever weird personal theories you might have on life will never satisfy you, keeping them to yourself (at least, they don't, me). there is hardly time to sit down for long discussions on one topic straight, and not everyone would be interested, anyway; so, at times, you have to "push" your points out when you're in the mood for it, where the interested can read and ponder while the non-interested would have the choice to skip it. 'cos nobody's gonna ask you for it if they don't know what you're thinking.