is very similar to "as a result." Something is a fact, and after
thinking about this fact, you can make a conclusion. It
sounds a little formal in everyday speech but not uncommon.
It's very frequently used when writing.
His skin burns very
easily; therefore, he's decided to stay under an umbrella and
wear lots of sunscreen.
Our lives have become too
complicated; therefore, we've decided to simplify things and
move to a log cabin in the country.
He spends too much time doing office
work--even when he's out with his girlfriend; therefore, she's
thinking about ending the relationship.
There were too many accidents on this road when the speed limit
was 55 miles per hour; therefore, the city decided to lower it
to 45 miles per hour. The limit for snowmobiles was also lowered.