This is a question of area and what is being done. For anything cosmetic (paint, flooring, kitchen cabinets, etc) you dont need a permit. And most boros around Pittsburgh don't even have an option to pull for that. When it comes to anything major like electrical, plumbing, or structural...I think you need a permit. And I know it eats up time, but honestly, those are big things. Switching out a toilet, no you dont need a permit (imo). Running new plumbing lines to a house. Yes. The answer as to why homeowners or investors choose to do unpermitted work is both time and money. It has to be done by the books. The construction company my husband and I own have a city license which is required to get a permit in the city of Pittsburgh. It is a pain in the *** to get and not cheap. The extra insurances we need to hold, bonds, etc. But, we do a lot of work in the city so it's worth it.
I will also tell a cautionary tell. My client was selling a duplex in the city and when they went to sell, they found out that the guy they bought it from did a whole bunch of unpermitted work and the inspector prior to the current one was his buddy and let it pass. Well now my client had to rip out all that work and re-do it with the correct permits of the city wouldn't release the lien letters. It cost my client tens of thousands of dollars. I say just do it right the first time!