A dirty filter gets talked about a lot in terms of system efficiency and airflow, and that's a real concern, but it's not the only one. The air passing through that filter is the same air circulating through your home, which means a severely neglected filter has a direct connection to indoor air quality, not just your energy bill.
What a clogged filter actually does to indoor air
A filter that's well past due for replacement loses its ability to properly capture dust, pet dander, pollen, and other particulates. Rather than trapping that material, an overloaded filter can allow more of it to recirculate through the home, or in some cases, air can bypass the filter around its edges entirely if it's clogged enough to restrict flow significantly.
Who's most affected
Household members with allergies, asthma, or other respiratory sensitivities are the most likely to actually notice the difference between a home with a well-maintained filter and one that's been neglected. Pets in the home also contribute more dander and hair to the system, which means homes with pets often benefit from more frequent filter changes than a general guideline might suggest.
This is separate from, but related to, filter MERV ratings
A higher MERV-rated filter captures smaller particles more effectively, but even a high-MERV filter loses its benefit once it's clogged with debris. Filter quality and filter maintenance are both part of the picture, a great filter that's never changed doesn't help indoor air quality any more than a basic filter does.
Have a question about your own system?