They are supposedly fitted to the return to protect the boiler, but no filter catches everything first pass, so especially with an older boiler, it really won't matter on the flow.
As mentioned, not a dig and understand why people do it this way. But, you can't really price a job without physically looking at it.
All that happens is you get a middle man who wants a cut and an installer who has to get done a quick as he can to make any money.
If you had used a recommended local tradesman rather than an online shot in the dark, this may have been raised earlier saving everyone a bit of time.
Not a dig, just saying.