What is the difference between .shrc and .profile?

I want to try out Bourne shell on FreeBSD so I am starting to set it up for my use. In .shrc, I set my prompt, enabled vi mode, set some aliases, and exported some variables. However, I see that .profile also, by default, exports some variables. It is my understanding that Bourne shell will source .profile on each startup. If so, what is the (historical) reason for having both .shrc and .profile?