This is a great question and a lot of things can be stated. In view of this, let me mention a few things.
First, all empires want to maintain their power. For this reason, they need to gain resources. More specifically, they need natural resources that are cheap and assessable. For this reason, there was a huge rush to Africa to gain its wealth of natural resources. The way this could be done is through trading and the setting up of good trade routes.
Second, empires, if there are many of them, are trying to gain the upper-hand. In other words, they are competing against one another. In light of this, many European nations sought to gain a part of Africa to gain resources before others. In short, there was a race to gain the wealth of Africa.
Finally, trade is tied to wealth. In short, empires need wealth. Hence, they were in the trade business, especially in maritime trade. Therefore, it is not surprising that Britain had a huge navy.