They only grant the right to exclude others from doing something.
So if i own a patent on X, it gives me the right to prevent others from doing X.
Even if i license my patent on X to you, it does not necessarily mean you can do X, because doing X may also require other patents that my patent is an extension of.
In other words:
Imagine we have patented processes, each building on the last.
Patent A covers doing thing one
Patent B covers doing thing one, then thing two
Patent C covers doing thing one, then thing two, then thing three.
Granting you only the right to perform patent C does not enable you to perform the process described by patent C, you'd still need licenses for patent B and patent A to do that.
They only grant the right to exclude others from doing something. So if i own a patent on X, it gives me the right to prevent others from doing X.
Even if i license my patent on X to you, it does not necessarily mean you can do X, because doing X may also require other patents that my patent is an extension of. In other words:
Imagine we have patented processes, each building on the last. Patent A covers doing thing one Patent B covers doing thing one, then thing two Patent C covers doing thing one, then thing two, then thing three.
Granting you only the right to perform patent C does not enable you to perform the process described by patent C, you'd still need licenses for patent B and patent A to do that.