In Breaking Bad, because Mike works for Gus Fring, presumably he should be all set financially. Also Mike is apparently loyal to Gus. Why would he need a part time job as a PI for Saul Goodman on top of his multiple activities for Gu's operation? This part-time job might even be a conflict of interests. Also Mike is obviously old and not very fit - why does he want extra work?
Why would Gus allow his loyal hit man do part time work on the side?
Mike has not spent a dime of Fring's drug money, which is revealed when he is first interrogated by the DEA. All Mike's earnings from Fring's employment are put away for his grand-daughter. Thus perhaps his PI work for Goodman covers his living costs.
Technically this might be a conflict of interest, but these are not moral, law-abiding citizens. Additionally, working for Goodman allows Mike to keep an eye on Walt and Jesse, who are very much part of Fring's interests. This would explain why Gus would allow Mike to work for Goodman. Now if Goodman were to ask Mike to tail Fring, Fring might have put a stop to Mike's extra work.
The only reason he would 'need' a job working for Goodman would be because it provides him with a legal income source. This allows him to 'hide in plain sight' like Mr. White and Mr. Fring do.
Him working for Saul helps his true employer, Mr. Fring.
Before Walt and Jesse entered the picture, we have no information about how often Mike did jobs for Saul. After Walt and Jesse became clients, continuing to work for Saul provided Mike with the opportunity to keep better tabs on the pair.
Saul likely works with lots of other drug dealers/junkies as well. Having Mike on Saul's payroll would allow Fring access information about them. Information that could be useful to Fring and his drug operations.
