IFTB, taking your questions in order...
Business presence, a place to put files you may want to get at from a different location, showing off, vanity and you can publish images on PPRuNe.
Mail forwarding yes; Web Site/Page obviously. A place to put your holiday photos.
Your Web Hosting Company (mine was Verio and is now 1&1 Internet Ltd) allocate space to you on their Server (for which you pay) and you place the files that make up your Web Site there. The is a simple rule that dictates what visitors see when they type in your URL. The file transfer is easy using "drag & drop" in Windows 2000 or XP. There are "File Transfer Protocol" (ftp) Programs that can be used. Basic files are composed in HTML using anything from Word 2000 (yuk) to Frontpage Express and other customised programs. Images can be put there as they are. Videos are fun and "gif" files are simple moving images that don't take up much space.
"What is the advantage?" - huh? advantage over what? The facility enables you to do something you couldn't do before. If that's what you wanted, then that's an advantage.
What experience...? Funny question. You get what you pay for and you reap the reward in direct proportion to the effort you put in. But isn't that true of most endeavours?
Reckon on spending about £50-£150 per year depending upon the level of flexibility and facilities you want from a hosting company.