Hi Gustavo, I'm no expert but I'll give it a shot.
Perfectly good brand-new tubes all have some variations in them. They can all be perfectly good, they just react slightly differently to the same input factors (current and voltage). Matching tubes insures that they all react about the same when presented with the same factors (loads, current, voltage, bias etc). The best analogy I can think of right now is car tires. When brand new and perfectly good, tires can be slightly out-of-round and need balancing, otherwise you run into some problems. While tubes can't be altered after production they can be matched at purchase and you can hope that they will perform consistently as a group. It's not the best analogy but hopefully it gives you an idea.
Is matching worth it? I suspect alot of people who sell "matched" tubes either don't really know what they're doing or don't have the proper equipment. And there are cases where matching tubes won't really make any difference. For instance, if your amps are single-tubed SET monos then those power tubes don't need to be matched. If you have a stereo amp with fixed bias then the two sides don't need to be matched. If your stereo amp runs cathode bias or multiple tubes are biased with one pot then you absolutely need matched power tubes. Sometimes small signal tubes need to be matched as well but this is less critical than power tubes.
Hope this helps!
