Applications of combline filters.

It is used to eliminate the higher order passband enegy.

klystron, I wrongly formulated my question. It should have been where can it be used?

Combline filters, due to using loading capacitors on the resonator lines, the total line dimensions are smaller compared to other filters as, Interdigital, Parallel Coupled, Hair Pin, etc.
Regarding the question, where can be used, the answer is: mainly in UHF up to few GHz circuits.