yes advertising /21 and /24 can be needed
it is not only because of your upstream which may not accept /24 it is also abour your internal network topology
it may be interesting for you to advertise locally doing hot potatoe routing to avoid transport in your backbone
bgp routing decision on remote side will be done with the longest path
advertising X.X.1.0/24 + X.X.0.0/21 in Paris and X.X.2.0/24 + X.X.0.0/21 in Frankfurt will make routers accepting up to /24 sending you trafic to X.X.1.0/24 in Paris and to X.X.2.0/24 in Frankfurt
if they only accept /21 you may get trafic for X.X.1.0/24 and X.X.2.0/24 in any point
Now they said, they have advertised my two blocks on /24 too.
Is there a way to check if they have done it ?
When I asked them, why there is no internet when I set my IP, they said, you need to do BGP routing inside yout router, then only your IP will be shown on speedtest or whatever.