The ability to select multiple sections of text at once is a lovely feature that could cut down on busywork when it comes to properly formatting your document. If you want to apply the same effect to three different words in a paragraph, you can simply select the text and apply your changes.
If you're using a Mac or Windows machine, selecting multiple text sections is very simple: select the first part of the text, then press either the Command or Control key, depending on whether you're using a Mac or Windows machine. Pick the other bits of text you want. You can change whatever formatting bits you want after you have selected everything.
It's a small thing, but if you have a lot of text to format, using this feature will be easier than using the format painter tool. The tool makes it so you don't have to set each parameters on its own, but it still requires a lot more clicking if you want to change a lot of text.
The feature can also be used for other things, such as cutting a few paragraphs, moving them around, or adding a comment that is relevant for multiple sentences. It's easy with multi-select.
There are a few situations where using the feature isn't the best option. If you're trying to change the style of your document, you'd be better off just changing it. You can format one of theheaders the way you want, then go up to the style section, select the level you want, and click Update to match.
It's great that there are some desktop app word processor that don't have multi-select as an option. If you don't have it yet, you shouldn't have to wait too long because the feature will gradually roll out to everyone's accounts over the next 15 days.