The short answer is that it's useful in cases where people mistakenly post meta questions on the main page. Beyond that it serves no purpose because no other migration paths have been defined.
The reason we don't request new migration paths is that by and large there is no need for them. You can read some additional info on the history behind the migration paths here but the short answer is that they were created at a time that migration was needed daily between closely linked sites. I expect migration is still uses relatively often at core sites due to the sheer volume of questions and the significant overlap, but for the less technical sites like ours, they make no sense.
One of the checks on migration requires that you verify up-front whether a question is of sufficient quality to migrate and that you are aware of the scope of the target site to know that the question would be on-topic. It is next to impossible to enforce this if users were able to vote on migrations, but it is possible to do quality control when only moderators get this ability and they can be held responsible for the migrations they do. Migrations are very rare as a result.
In addition the number of paths is limited so we couldn't possibly cover everything even if we wanted to set up migration paths and migrations themselves are so rare that it doesn't make much sense to go through all that trouble. Allowing user-led migration to any site has also been discussed before.
The recommended approach remains to flag a question with a custom close reason recommending a migration to another site. Keep in mind that the vast majority of these flags are declined. Only rarely have I run into a high-quality question that was off-topic here but on-topic on another site.