Spider Web, a move only learned by 3 fully-evolved Pokemon species, is an obscure trapping move with no additional effect. At least, that's all it does on-cartridge. In BBP, the move takes on additional effects and special interactions, becoming a unique attack worthy of its rarity. Here, not only does it prevent the opponent from switching out, but it also binds them, preventing them from using Contact moves until the effect wears off or until they use a Fire-Type attack to burn down the webbing. It's a versatile defensive move, especially in Singles battles.
Galvantula is an excellent practitioner of this move. Its impressive 108 Speed allows it to out-speed many of its opponents, tying them up with its webs before they get the chance to strike. Then, while the opponent is unable to swap out, Galvantula can use Volt Switch to turn the matchup completely in your favor! Depending on the timing of when you use Spider Web, it might be several rounds before the opponent has a chance to retreat.
As for the other effect of Spider Web, one potent use for it is to prevent the opponent from using Rapid Spin, a Contact move. Both Galvantula and Ariados have access to the same hazard-setting move: Sticky Web. As such, they both appreciate having the power to prevent the foe from Rapid Spinning hazards away. Ariados, in particular, benefits immensely because it also learns Toxic Spikes.
Spider Web, however, does have downsides. For example, its duration is halved against Bug-Types. It's susceptible to Taunt. Pokemon that have Spider Web as a known move are completely immune to it. And, worst of all, the webbing can be burnt down by Fire-Type attacks, which Bug-Types are weak to! So, don't expect yourself to protect against Flare Blitzes and Fire Punches with this move.
If you're the type of person who enjoys pinning their opponents in sticky situations, give the Spider Web move a spin! Joltik, Spinarak, and Dewpider all learn it, and all three are available as starting Pokemon.