What I like about topical is that if you want, you can take much less of the drug much more easily than with the oral form. Applying less of a topical compound is much easier than cutting a 1mg pill into small pieces, or however else some people take smaller doses. I can see it being easier to taper up on topical. (There is some debate about the proper dosage for the drug, so for people who think they can get benefits with less of the drug, it might be a good option.)
Also worth noting that although there is variance in the vehicle and whatnot, any compounding pharmacy can make topical finasteride.