Why accept tonnage limits at all?

What benefit would the Soviet Union gain from accepting naval tonnage limits?

It would gain influence over rules that already constrained Soviet planning and oppose a settlement that admitted Germany as a major Baltic naval power.

Treaty limits already shaped Soviet choices because exceeding them could trigger escalator clauses and counterbuilding. Signing would give Moscow a voice in the rules that constrained it from outside. Moscow also feared German admission to the treaty as a major naval power in the Baltic, where Soviet Ganguts already operated. Participation would allow Stalin to contest that outcome and influence the terms.