There is an innate urge in all of us to communicate to other human beings. It is a matter of belongingness. Very few people seek absolute isolation and a non-communicative way of life. The true recluse is rare. So what better way to communicate, be it a one-way broadcast, than to write on a site like Hubpages.
On Hubpages there are very few restrictions on what we can write about, and there is the potential of thousands of readers. Not that many of us get thousands of readers, but the opportunity is there.
I guess there are two sides to it: our ego side of wishing to be recognized for what we're capable of doing (or think we are capable of doing) and the REAL self desire to put stuff out there that will actually help other people. These question-and-answer forums certainly fall under this last category, for you can often see people go out of their way to give wise explanations and good advice to people they will probably never meet.
But, to reiterate, there is a need in us to communicate and, in the 'writer' that need can be very demanding. As Kathleen says above, writing can have a strong catharsis effect. It also clarifies the thought and philosophies of the writer. So that's a couiple of the reasons I write.