Subaru Boxer engines have little tolerance for sludge caused by missed servicing changes or "supermarket grade oils". Every second service the Subaru equivalent of Seafoam is put through the engine.

If you don't service your car properly or skimp on premium grade components fuels or lubricants or run it infrequently, then there maybe a case for running some additives through.
If you buy a vehicle with unknown history and shows signs of sludge, carbonizing or gummed up fuel components then a case for putting cleansing additives through your vehicle maybe there.

M.B. advise Techron as a fuel additive. This is already added to Chevron or Caltex premium fuel.
Nothing wrong with the occasional "Italian tuneup" occasionally

At least one re-builder of early model fuel distributors states that the overuse of harsh fuel cleaning additives is detrimental to the F.D. internal gaskets and the cause of premature failure.