There's a couple of obvious advantages to using Freecycle (or Freegle, which is the new UK-based version). Giving a home to things you don't want or need any more, rather than throwing them away. Getting hold of things second-hand -- and free! -- rather than having to buy new and generate more waste. (I got a stairgate from Freecycle recently when we acquired a new dog.)
But I've found that it also helps with the process of deciding whether you really need to keep something at all. I've been trying of late to move away from a policy of "keep it just in case". As a policy, that leads to stacks of belongings festering in corners; reducing the space available for you and for the things that you genuinely do want and use.
Freecycle lets me have the attitude that if I need something at an unspecified later date, I'll be able to get hold of it again at that point. If I send out into the wild the stack of paint trays and rollers that have been in the bottom of a cupboard for 5 years, then should I ever need them again, I'll be able to find another set.
Of course, that particular set of paint trays may never be in circulation again. But the more stuff there is circulating in the free and second-hand un-market, the more likely it is that the stuff you need will be there when you need it.
I've started to see "keeping things just in case" as a form of wastage. It means that a useful thing isn't in use, so when someone else needs it, they have to buy another one. As opposed to using the one sitting unused in my cupboard. In a similar vein, I share a bike trailer and various power tools with other people: they're expensive things that we don't all need at once so why own multiple versions? I can treat Freecycle and free shops as something a bit like a large and less trackable version of a lending library. End result: less stuff in the world and in my house.