Why is engineering required for serious raiding guilds in Vanilla?

I’m just curious as I’ve joined a pretty committed guild for this fresh cycle. I’ve never played Vanilla but I have played Wotlk for a solid 10 years on [REDACTED] servers. They are requiring all raiders to have engi, and I see most guilds have this same requirement. I’m just curious what exactly does engi offer in vanilla to make it so important? Like wrath you have hyper speed accelerators, rocket boots, etc.. What’s so important in Vanilla for raiding? Sappers?