Floor safes offer inherent protection against fire, water, and theft. Thiefs don't spend lots of time looking for safes in the floor, and even if they do locate it, they are unlikely to be successful breaching it.
I have to admit that I have never been a burglar so I am just guessing here, but I would say that most burglars just aren't really that motivated to spend a whole lot of time in someone's house or place of business while ripping them off. They don't really want to spend a lot of time looking around for things like floor safes or other hidden safes. Nope, they prefer quick in and quick out. I'm sure most crooks don't mind a quick check behind the paintings, or in bookshelves, or behind cabinet doors. But beyond that, being in someone's house uninvited, with nefarious intention, just isn't a leisurely la-dee-da experience. So, using a floor safe increases the likelihood that a thief will not be successful just by the nature of the safe itself.
However, using floor safes to protect valuables has both good and bad aspects. On the one hand, they do go beyond what a normal safe does, in terms of traditional protection. Because they are in the floor they are inherently hidden and, therefore, more secure. They can be situated in the floor wherever you want if you have the opportunity for some advance planning. Because floor safes are usually installed right in concrete it is best if you can identify a location before the concrete is poured. This keeps cost down and also allows you to opt for one of the larger safes.
On the other hand, floor safes that are not installed at the time the concrete is poured are more expensive to install after the fact, but can still be used. Manufacturers of floor safes are aware of this and make smaller round canister-type safes that can be put in for less cost than a larger rectangle or square one. Another thing that can be done is to forego installation in concrete and just install it in the floor joists. This can still be a good option, but you will give up some of the great security benefits. Depending on your needs, this may still be acceptable.
Floor safes have several other advantages over traditional safes as well. Because they are below the floor, they can survive high temperatures more successfully than if they were in a wall or just sitting on the floor. This is especially true if the safe is encased in cement on all sides except the door. Because the door is designed as the most secure, thickest part of the safe, and the rest is below ground, floor safes are more likely to survive fires. You can purchase safes that are specifically rated to protect the contents inside from fire, but because of the way floor safes are installed, they give you the the added benefit that fire safes offer even if they don't have a specific fire resistant rating.
Some floor safes have the additional advantage of having seals that protect against water getting in. But even with this added feature, most manufacturers recommend that a waterproof bag be used inside the safe for maximum protection of the contents. Though some manufacturer have even designed safes that don't have hinged doors at all, the door has to be lifted out in order to remove what's inside. I like this feature because it would take away one of the biggest vulnerabilities (water damage) of this type of safe. So, consider floor safes for maximum security and protection of your valuables.