As long as you keep a pc with the specs and OS of the time the game was released. GOG is also making an effort to patch these games to make them run on current hard- and software without the hassle of finding and downloading fan patches, running emulators/virtual machines and all the other hoops one might have to jump through to get an old game running.

Of course you could theoretically pirate the gog version after they made it run, but given that these games usually cost about 5-10 bucks and some go as low as 1-2 when on sale, i think that's worth it to support these efforts.