The main difference between them is 15 character IDs are case sensitive, whereas 18 character IDs are case insensitive. It becomes necessary to convert IDs from 15 characters to 18 characters when you work with vlookup functionality in excel. Using below tool you can simply convert IDs with character length 15 to 18. Just copy all 15 character IDs one on each row, paste in text box, hit on convert now. You will get list of converted IDs in second box.
Hope it helped you. Don't forget to share this tool with other admin/developers as well.