As always, the answer is relative, and depends on the entity of the reform we are going to undertake.
If we are talking about changing the tiles in the kitchen, it will suffice for a mason and, perhaps, a plumber to readjust the position of the faucets. For this you do not need a contractor; actually, the mason himself will be acting as a contractor, and it is very likely that he will bring the plumber of his choice, with whom he usually works.
If, instead, we are talking about a profound reform of housing, involving different professionals (electricians, masons, plumbers, painters, etc.), it is a good idea to hire a contractor to help us in the execution of the work. Of course, if we think about the construction of a new home, it is the law that imposes on us the obligation to have certain professionals who certify that the constructed building complies with what is established by the local and state laws.
If you already have experience in some works and in deep reforms you may think you can do it without the contractor and conduct the operations yourself, but even in this case, it is advisable to put yourself in the hands of a professional for the following reasons.
The legal aspects
Although you have seen dozens of times a working carpenter, you may not be aware of the laws on woods (the use of some tropical woods is forbidden, for example) or on paints and other chemicals. The same happens with electrical or plumbing installations. A construction professional will be up to date on the rules on his work, and that knopwledge will prevent him from being penalized, just to maintain his business. You can not be aware of all the legal aspects of any work of a certain level.
In addition, the professionals usually protect themselves with liability insurance, in case something goes very wrong, and probably the cost of these insurance is greater than the price you would pay to have the services of a contractor.
Want you to be an employer?
Working with employees is very complicated. If the general contractor does not exist, you become an employer. Do a search on the Internet to find out how many well-intentioned homeowners, who thought they could handle their small construction, have been plagued with problems for the rest of their lives due to compensations or other labor problems.
If you hire a general contractor, you become a consumer, and thus laws are in your side.
Availability of professionals
If your electrician fails, it will take you a long time to find another one. Meanwhile, the work will be frozen (and time is money!). Or you will have to pay a very high price to have an immediate replacement. Contractors have a wide array of professionals to avoid such setbacks.
Make your numbers, and in the end you will see how it is profitable to pay a little more to not assume all the risks that a work entails.