The SPUs were basically the next evolution if the ps2's two vector units, which were often running middleware too. It was really hard to write code that ran fast (Sony were advocate of writing assembly in an excel spreadsheet), but generally the vector units were doing about the same thing for every game/developer (bruteforce stuff: transforming vertices, calculating vertex lighting, generating multi-pass command streams for the rasterizer). So most developers were just using Sony provides examples, or improved versions from middleware developers. Very few developers wrote their own VU programs, or even needed to.

The cell now has 7 vector units, with comparatively more memory, but there was no default job for them, all the vertex transformation now ran on the GPU's vertex shaders. And Sony initially stuck to their guns of "SPU programs should be written in assembly, in a spreadsheet"

Because the single PPU really sucked and was nowhere near fast enough for anything, Sony eventually relented and releases a version of GCC which would compile c++ code to the SPUs. Fast to develop, but nowhere near the performance of an excel spreadsheet designed SPU program.

This resulted in a whole bunch of games running code on the SPUs that was really badly optimised. But at least it reduced load on the PPU.