It's not extremely important for most people, but here (Netherlands) it's mandatory to stay within the mask.
Sorry for the off-topic but I have never seen an option to enable or disable the stokkemask on any other processor. Does it mean that all the Omnia / Orban / Wheatstone boxes always keep the signal within the mask? Or do they require further adjusting to be used in the Netherlands? Are you aware of other countries where you must use stokkemask?
Breakaway has it, as far as I know, nothing else does. Which is why you will see that most of the big stations in the Netherlands aren't modulating at 75 kHz, but often around 68 kHz or so. (The AT started to enforce it more recently). Just to stay inside the mask. Also, with composite clipping it happens more often that you go outside the mask (since the signal is louder).
By the way, without composite clipping it's completely impossible to know whether you're inside the mask, unless you stay very far below it, because the pilot, pilot phase and RDS all affect the signal. (Which also means that it's a bad idea to add RDS after the processor).
I think they use it in Belgium as well. Beside that, it's an ITU recommendation - and it supposedly improves reception. Receivers typically bandpass-filter the signal, and the more narrow their filter is, the more of the sides it cuts off. So anything you're broadcasting there will interfere with other stations on nearby frequencies - you're basically putting part of your signal strength at a place where a receiver that's tuned to your frequency isn't listening. And, the cutting off of the sides in turn affects the audio quality, and also measurements that receivers do to determine noise levels etc, which will lead to mono blending and all kinds of stuff like that. It also greatly reduces multipath issues - enabling Stokkemask in Stereo Tool has a twice as big effect on the RF bandwidth used as techniques such as SSB stereo.
So basically, turning Stokkemask on is *always* a good idea, but until now my filter was affecting the stereo separation too much at very high loudness levels (mainly on 8 dBm+). That issue is mostly gone now. (There is still SOME effect if you feed very wide stereo - that's unavoidable).
I often get feedback from stations that switch from another processor to Stereo Tool that their reception area - and especially their stereo reception area - gets a lot bigger, in some cases (100 kW stations in the US with no nearby stations at the same frequency) the stereo reception area increases by 20 miles in all directions. This is part of the "secret sauce" that's causing that
Actually, we just had a Dutch regional broadcaster that reported that their reception area increased, so apparently even in the crowded Dutch FM band it helps.