Eyebrows were raised when Liverpool paid £17m for the right-back but Benitez has explained why it was money well spent.
"Sometimes you have to spend a little bit more," said the boss.
"You can't compete in the top four of the Premier League unless you spend some money.
"We were looking for a player of quality and also somebody who is British because of the new Champions League rules.
"Glen Johnson fits both categories."