These end caps are designed to hold the roller brush in place while rotating. Usually the main reasons why your brush bar is no longer spinning, or not spinning freely is because hair or debris gets inside it and may be seized (hair melts, plastic melts).
Very easy replacement, just pull out the old end cap and put in the new one. Make sure to clean the roller shaft from any debris or hair accumulated inside.
SEB-217 powerhead currently is included with Miele C1 Cat & Dog, Miele C2 Cat & Dog, Miele C3 Excellence, Miele Bagless CX1 Cat & Dog.