According to Marx and Engels, classes are always in a struggle against one another for power. This class struggle, they say, is always a political struggle. The bourgeoisie, then, wants to gain political power so that the interests of that class can be protected.
Each class has its own interests. The interests of the bourgeoisie are tied up with promoting and expanding capitalism. The bourgeoisie want to spread capitalism throughout every country. Once it is in place, they want to keep it that way because they are the class that is in control in a capitalist society.
Therefore, the bourgeoisie want to gain and keep political power. They want to do so in order to achieve their class goal, which is to have a capitalist system in which they are the controlling class.