From wikipedia:

The term "tankie" was originally used by dissident Marxist–Leninists to describe members of the Communist Party of Great Britain (CPGB) who followed the party line of the Communist Party of the Soviet Union (CPSU). Specifically, it was used to distinguish party members who spoke out in defense of the Soviet use of tanks to crush the Hungarian Revolution of 1956 and the 1968 Prague Spring uprising, or who more broadly adhered to pro-Soviet positions.^[7][8]^

Basically it's because certain western communists defended the use of tanks by the Soviet Union to forcibly crush rebellions, hence the term "tankie". I think it's useful to have the term because they're a distinct phenomenon from regular fascists in that they at least pretend to be leftist and thus tend to use different talking points and arguments — even if it's to achieve the same fascist goals.