They say you should never use the email address provided by your ISP as your primary email since ISP email addresses are not transferable between ISPs (as phone numbers are in most countries).  Unfortunately there are some times when you can't avoid using provider email such as on mobile phones, so if you ever decide to change providers you can take your number but you'll still need to inform everyone of your new mobile email address.

Another drawback specific to mobile phones is the limited storage on the email servers and devices.  Vodaphone (now Softbank) originally provided 2MB but has since upgraded to 3MB which is still pathetic and makes archiving mail impossible.

That's when the idea hit me to use Gmail for ALL my mail, even mail mobile mail tied to Vodafone (now Softbank). 

What the "routing" does

I use the term "routing" in quotes because this is not true mail routing but a sort of user level routing.  Instead of giving out the email provided by your mobile carrier, you give out your.email+mobile@gmail.com.  Notice the use of the + addressing.  This is a Gmail trick which allows you to create unlimited unique email addresses with your existing account.  Anything after the + is ignored for delivery purposes so Google still sends it to your inbox.  In this case we'll be using the extra text after the + sign as "routes".  When someone sends mail to that specific "+" address, Gmail will deliver it to your inbox but also forward it on to you@yourmobile.com.

Now you can take advantage of the huge storage and other features of Gmail for all your mail.  Also, if you ever change carriers you don't need to inform anyone of the change.  Just change the forwarding address in your Gmail settings.  This makes it a lot easier for your contacts and for yourself as you'll see later.  

How to set it up in Gmail

1. Think of some "routes" for your destinations, for instance "mobile", "twitter", "work" etc and insert the route name into your address <username> + <route> @gmail.com for each of the routes.  For instance your.email+mobile@gmail.com.
2. Enable Gmail's filtering feature to catch emails sent to the "routed" addresses.
3. Set Gmail's forwarding feature to forward all emails bound for a specific routed address to the actual destination email.  In the example above, all email sent to username+mobile@gmail.com would be forwarded to you@provider.com.

Other uses for "routing"

You can also use this to simplify those long, cryptic email addresses generated by Email API systems.  For instance <username>+microblog@gmail.com could be forwarded to your Laconica posting Email address to post microblogs.  If you do this, don't share the simplified address with others or they will be able to post as you.  Note: This only works on systems that don't check the return address because the envelope will not be re-written.

Don't forget, you should change your "reply-to" address on the devices from which you send mail.  Unfortunately, some devices don't allow you to change your "reply-to" address so replies may still display your real mobile address.