Besides what you mentioned with "I need the money now", the other main reason is the time factor. You can work a few nights a week delivering pizzas if you need some money, where as I do not know of many people that have a profitable small business operating only a few nights a week.

To me a second job is a temporary thing where you have a lot less hassle initially but not as good financially long term, compared to a side business where you have a lot more hassle and set up time initially but long term can be a bigger financial advantage.

I think there are a few "second jobs" that can qualify as a side business. Think about lawn maintenance, if you already have a lawn mower, blower and weed-eater, all you need is a license and a willingness to find a few clients-and you get paid by the job.

Other similar, low overhead quick return options would be tutoring, baby sitting, and house/office cleaning services.

Online businesses frequently have low start-up costs, but take at least a 3-6 months, and more likely a year to begin yielding any significant income, while you build traffic and learn the ropes.

One of the biggest hassles of starting your own business is the whole legal side--reporting taxes, getting licenses, complying with regulations, etc. Even something as simple as a candy machine route can get very demanding if you try to be legal. If you don't want to deal with such issues, then taking a second job and letting someone else deal with it instead would probably be more attractive.