What effect did Pokemon have on the metagame?
Before it was banned, it would show up on almost every team. It served as glue for many team types, was the poster child for good removal, was bulky even when eviolite was removed, and had multiple options for preventing overly stall reliant teams. It overall outclassed its nearest equivalent in Vullaby and prevented it (as well as other flying type defoggers) from having much of a place in the metagame simply by opportunity cost. Its overuse paved the way for a metagame where some of the most important attack types were Psychic, Ice, and Electric, which paved the way for Elebuzz, Piloswine, and, at various points of time, Pikachua, Kadabra, Galarian Mr. Mime, and Duosion to be important to the metagame.
In what main roles was Pokemon used?
The most common role it served is being a solid defogger (and pivot). But it also had other important sets such as stallbreaker with Super Fang and Taunt and Nasty Plot sweeper, both which can push an opponent expecting a defog or U-Turn.
What caused it to have a significant impact?
The versatility of moveset, amazing bulk with eviolite, and amazing typing which makes it immune to toxic and grounded hazards as well gives it an important immunity to Ground-type moves. Especially after Kadabra and Galarian Mr. Mime were banned, it could very safely U-Turn out on most of its threats barring Electabuzz.
How do/did you deal with this Pokemon in NFE?
One of the most important ways before most of these were banned was finding opportunities to get Kadabra, Galarian Mr. Mime, Piloswine, or Electabuzz in to threaten Golbat out. Over time, you can wear down the portions of your opponent's team which cannot recover to remove their ability to easily switch out Golbat. Of course, Knock Off was helpful, although with occasional Thief Golbats, that wasn't always helpful. In terms of hazards, it was helpful to stick with Stealth Rocks, as any other hazards made it very safe for Golbat to set up on. But ultimately, taking down a Golbat was frequently a struggle even for prepared teams.