Jane may have been in the habit of being in a more diverse society in London, but in Highbury she lives in a small apartment with her grandmother and extremely talkative aunt. Emma's friendship would offer more invites to Hartfield, more variety of location, etc.

I can imagine that an excuse that "I must go and call on Miss Woodhouse" could be a pleasant escape from home from time to time. At times a change is as good as a holiday.

I think also you are a little quick to say that Jane is so much Emma's superior that all the gain in conversation would be to Emma. I learnt many years ago not to dismiss people from my consideration as being inferior to me, even the most unlikely people have reservoirs of surprising knowledge and experience. Just because Emma did not have the advantages of London masters and theatre does not mean that she is doomed to bore Jane. Emma is intelligent, witty, and can laugh at herself. Emma also has the experience of being mistress of Highbury, we see her visiting an old servant so she keeps in touch with a range of people, she's active with the cottagers. Jane may know more about London life, but Emma surely knows more about rural life.