At the time the Soviet Union had the Spanish government over a barrel because they were the only people who could/would supply significant quantities of desperately needed weapons (the opposing Nationalists were better equipped for reasons I'm not entirely sure of).

This also contributed significantly to the Communist (Stalinist) faction winning the internecine struggle on the left against the Anarchists and Trotskyists, but the quid pro quo was loyalty to the Soviet Union.

In hindsight we know they lost anyway, so this seems really silly, but at the time everyone involved believed they were in a completely existential struggle against fascism that was more important than literally anything else.

For anyone interested Orwell's Homage to Catalonia is an excellent book although it's just one man's perspective and may not be entirely historically accurate