A dispensation is for times when it is possible, but you are requesting the obligation be dispensed for a just reason.
To give a concrete example: 6 years ago I participated in a charity long-distance bike ride (Montreal to Quebec City in 2 days), to raise funds for cancer research (google “Ride to Conquer Cancer”), on behalf of the Montreal Jewish General Hospital which is a top-notch cancer research center in Canada.
The ride started bright and early on a Saturday morning. We camped in Trois-Rivières (small city half way). Obviously we had no transportation other than our bikes, and after riding over 145 km that day, were in no shape to find a Saturday evening Mass. Similarly, we hit the road bright and early Sunday morning for another 145 km.
I asked for a dispensation and since it was for a good cause, was granted it. Theoretically it
was possible to find a Saturday evening Mass in Trois-Rivières, but feasibly, it would have been difficult to find a church, and ride and safely park our high-end road bikes with no facilities to lock them; moreover the ride rules required us to park our bikes in a secure compound upon arrival at the camp. As we were camped on the outskirts of town, it was not easy to find a church to walk to. Moreover after 145 km in the saddle we were more than just a little tired.
The ride was a great experience BTW, and I bumped into a couple of friends and former colleagues who also happened to be riding it. The toughest part was not the ride as we were well trained, it was coming up with the $2500 in sponsorships each ($2500 for me and the same for my wife).