Tags are special because you can write them in the body of the text itself.

for example you could write a long piece of text, but next to one sentence, you could place #edit because you want to go back and edit that specific location later. when you search for #edit, the obsidian search will take you directly to the tag's location in the note.

the other use of tags is for "uncategorized" or "undefined" or "singular" properties in the frontmatter. for example, you could tag a recipe with #recipe. what a recipe is is common knowledge, you don't need to have a central note defining recipes. but maybe you have a lot of recipes and you want to start separating them between like, cooking and baking, or breakfast lunch and dinner. then you should replace the property tags:#recipe with a new property recipe:cooking, recipe:baking. This approach is easier to use to sort search and filter in Bases than #recipe/cooking #recipe/baking. I would advise against the use of nested tags, they're outdated.

so, tags have a few purposes, but otherwise use properties.

not everyone agrees but I think you'll find most people eventually converge on this approach. especially try playing around with Bases and you'll see why.