Schlieffen has Pommern Secondaries, so the 105's will pen 26mm. In most cases their main function is to light fires and they do this extremely well given how fast they can shoot if you're Full secondary build. If you take Pyrotechnician as your first skill (which you absolutely can here) then your 787 secondary hits means (roughly) an extra 8.87 fires for the match. That adds up to a LOT of extra damage on things like Battleships.
They will also absolutely shred DD's that are within 10km.