If you still cannot picture out how they look like, some people use them to portray smilie that is smiling rather than a colon and a closing parenthesis.
To use an umlaut, do this: touch and hold down the letter on the keyboard. An array of accent marks will appear. From there, your beloved umlaut will be there.
This is a neat tip from Tom’s Guide. Probably not very useful to many people, but definitely in some cases.