Well, why not should be your question!  Here you’ve invested in a web site hoping someone will visit it.  Great!  But maybe you get them there but why would they return?  If you get them to sign-up on the page, you now got an audience you can target!  Or you’ve got an audience you can offer a better level of service to.  Store their login’s in a database along with any type of other information you would like and that they are willing to give you.  Now instead of you just being a vendor to them, you’ve got the beginnings of a partnership with them.  They’re trusting you to use their information with prudence and not overwhelm them with garbage.  Time to update the Term’s and Condition’s and Privacy Policy of your web site.