The SNP are going to get enough seats in the constituency votes that they'd be very very unlikely to pick up many seats on the list. (Which was also the case in the last two elections)

By standing on the list only, people can vote SNP locally, and then for him on the list, and he won't have to get a very high percentage to get some more pro-Independence MSPs, without it actually costing the SNP anything.

List-only parties that are basically the same as a well-supported party are the failure state of AMS.