[leWrat]

Some people are happy to take the risk that any likely tire damage that they get can be fixed by the goo/gunk, whilst others prefer to know that regardless of the tire damage they can swap it out for the spare and continue with their journey (albeit maybe a bit slower than before).
I would add that I think that you can only repair a tire once with the goo meaning that you should probably replace that "repaired" tire at your earliest opportunity.

My experience is that I have only once had a side wall damaged, but I have had more than one repairable puncture in the same tire on several tires.
In both cases I was able to use the spare wheel to continue my journey and later go to the tire shop to replace the tire (for the first case) and get the tires repaired (for the second cases).
So for me it is well worth my peace of mind and also a cash saving to have the spare tire.