A snuffle mat is a piece of fabric with strips tied into it, and you hide dry food in the folds so your dog has to sniff it out. It looks like a shaggy rug and it costs more than a shaggy rug, which is why most people arrive at this question suspicious.
Short answer: yes, for the specific thing it does, and no for several things it is often sold as doing.
Why sniffing tires a dog
A dog's sense of smell is not a slightly better version of ours. The part of the brain devoted to processing scent is proportionally around forty times larger than the equivalent in a human, and a dog has a separate organ for detecting chemical signals that we do not have at all.
Running that system hard is work. This is why scent detection dogs get short shifts with long breaks, and why a dog who has spent fifteen minutes searching will often go and lie down afterward in a way they do not after a walk of the same length.
There is also a measured effect on stress. Research on shelter dogs has found that scent based activities reduce stress behaviors and produce more relaxed body posture, which matters because shelter dogs are about as stressed as dogs get. Nose work is one of several kinds of enrichment, and it is the one with the most evidence behind it.
What it is genuinely good for
Slowing down a fast eater. A bowl goes in fifteen seconds. The same food in a mat takes ten to twenty minutes, which is better for digestion and for dogs prone to bloating.
Settling a dog without space. Small apartment, rainy day, an injury that rules out walks. The mat does not need floor space or your participation.
Recovery after an operation. When a dog is on restricted exercise, nose work is one of the few ways left to tire them out.
Quiet. A dog on a snuffle mat makes no noise, which matters if you live above someone.
What it does not do
It does not replace a walk. Your dog still needs to get outside, see things and use their legs.
It does not fix separation anxiety. An anxious dog usually will not eat at all once you have left, so the mat sits there full.
It does not cover the chase instinct, which is the one that produces a dog standing over you with a ball at four in the afternoon. Sniffing and chasing are different drives and satisfying one does nothing for the other. If that is the behavior you are trying to solve, a ball that moves on its own is the thing that addresses it, and the range of interactive balls covers a few different styles of movement.
And it wears off. A mat used every day at the same time in the same place becomes routine within a couple of weeks.
How to use one so it keeps working
Start easy. Food sitting on top of the fabric, so your dog understands there is food in this object at all. Push it deeper over the following days.
Use their actual meal rather than extra treats, otherwise you are adding a few hundred calories a day without noticing.
Do not use it every single day. Rotate it with a puzzle, a scatter feed on the lawn and a rolled towel, so it stays interesting.
Supervise the first few sessions. A small number of dogs try to eat the fabric, and those dogs need a different toy.
Wash it. Kibble grease builds up in the strips. A machine washable mat matters more than it sounds like it should, which is why the BorkBrain Sniff is built to go in the machine and come out flat.
Is it worth buying, or should you make one
A homemade mat is a rubber sink mat with strips of fleece knotted through the holes. It works. It takes a couple of hours and about the same money in fleece, and homemade ones are usually harder to wash because the knots hold water.
If you enjoy making things, make one. If you want it working this afternoon and coming out of the machine clean every week, buy one. The activity is identical either way, and the activity is the part that matters. If you are choosing between this and the other mat people recommend, snuffle mat against lick mat sets them side by side.