You may not intentionally use tags to search, but the site's search function does make use of tags, so yes, it is very important
Example; SKU is considered a tag, and if you search an SKU only, you will get that product as long as it actually has an SKU and it was properly entered when posting.
Also, categories are treated as tags as well. The user-defined tags simply expand on that.
Lots of people would search on say, "wardrobe" or "G8F" or they may be looking for all items by "SomeArtist". These are only available if the uploader added them as a user tag.