According to The Communist Manifesto, why did the bourgeoisie want to gain political power?

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.