In the "Whole Cake Island" arc, Luffy and his crew were having a fierce battle with the Big Mom Pirates. However, they didn't ask or intend to get help from the Straw Hat Grand Fleet.
The alliance would surely have helped make their mission to retrieve the Poneglyph a lot easier and the Grand Fleet swore to protect Luffy, so they would have been great assets, especially in the final battle.
Why didn't they ask or intend to get help from the Straw Hat Grand Fleet?
The Whole Cake Island mission was never supposed to be a war against the Big Mom pirates, though it turned out to have significant fighting. The main aim was to retrieve Sanji as quickly as possible and then head to Wano. Robin only asks Luffy to copy the Road Poneglyph if they find one / get the chance to.
If you recall the end of the Zou arc, both Law and Zoro mention how dangerous and risky a move it is to be confronting Big Mom while they were already set to clash with Kaido. However, Luffy is adamant about getting Sanji back, and hence they split up. Not to mention the Grand Fleet, not even the entire Straw Hat crew headed to WCI.
Like you've mentioned, the Grand Fleet could have been of use in a number of situations, whether during WCI or the coming arc in Wano. However, Luffy doesn't really approve of the fleet at all, as summarized in the wiki:
Luffy rejected their offer, as he wanted to sail freely on the seas without having to command anyone. In response to this, the crews made the decision to still serve Luffy, but only when he needed them to, and consumed sake cups to form the Straw Hat Grand Fleet.
He'll probably only rely on them when they're around, and the situation forces it somehow. That's just how his character has been developed. Further, Oda has teased that the final ending of One Piece will be a war so great it would make Marineford look like filler. Many people expect it to be the arc where the Straw Hat Grand Fleet will be on full display.
