So far these two characters were berated by Sentai fanboys as being "unnecessary" and I do had that point of view too, since in Tokusatsu, characters like these either work alongside the heroes knowing their secret identity to becoming the heroes themselves. So I think I'll talk about how they got involved in Power Rangers.
At first, they appeared to be two bullies who weren't exactly much of bullies either since they didn't do drugs or beat people up. They normally just came with wacky schemes to make money which included trying to figure out who the Power Rangers were. I just hate the fact that Skull has a crush on Kimberly. Their attempts to find out who the Power Rangers were always a source of comedy.
In MMPR season 3, they became junior police officers to detectives. It was a big change for them. I kind of compared Lt. Stone to Detective Hayate in Kamen Rider Black RX.
In Turbo for some reason, they were changed into monkeys. This is plain silly if you ask me. Later on they had various odd jobs.
In Power Rangers in Space, the two joined in with Professor Phenomenus in various crazy plans. They did play a big role in encouraging everyone to stand up against the enemy and supporting the rangers no matter what. However the duo was later separated in Lost Galaxy.
In Lost Galaxy, it's shown they have become respectable businessmen but they only appeared for a short time.
Bulk in Power Rangers reminded me of Raita/Yellow Owl from Jetman because they're both funny characters. Hmmm if Saban got Jetman instead, Bulk may have become a Power Ranger and later Tor the Shuttlezord. LOL.