They are necessary when there are telecoms equipment plugged in to the line that haven't been designed/tested with the additional high frequency signals that deliver your DSL (e.g. mismatching equipment can cause 'reflections' back on to the transmission lines). I found that most of my phones don't impact the DSL connection, but one (a Siemens Gigaset cordelss phone) caused the DSL synch to fail when the phone was picked up or put down. I put a filter on that and the problem is solved. The other phones remain filter free.