Bamboo rods were originally made with two tips due to the types of glues and varnishes they used in making the rods years and years ago. Moisture would get into the glue and soften them some, making the rod tip a bit weak after a lot of exposure to moisture. Folks would fish from morning to lunch and then take a break to stretch out their silk lines and redress them and then fish with the second tip for the second half of the day with their dry and dressed line.
This hasn't been a problem for many decades now with the modern glues and varnishes used.
Some, not very many, builders made rods with two different tip tapers for different actions. Some even made rods with different lengths, but these were even less common. The Orivs Midge/Nymph model and the Orvis Pace Changer model are probably the most common of the different length tip rods around.
2 tip rods that are built these days are basically because that is what the buyer now wants because that's the way it use to be.
A second tip can save the day when you are clutsy and break your tip-usually in a car door or window or walking into a tree.
As for switching tips mid-day to avoid putting a set in the tip due to normal casting and fishing (on rods made within the last 60-80 years), that is a bunch of crap. Normal fishing/casting will not put a set into a bamboo rod anymore than it will a graphite or fiberglass rod. Improperly landing a large fish is the most common way to put a set into a bamboo rod.
Many bamboo rod users do alternate tips every other fishing trip just to wear them evenly. But if they don't, it will not have any negative affect on the rod or it's action.
Bamboo is not this delicate fly rod material that everyone thinks it is. Much stronger than any other material fly rods have ever been built of. The problem is when a rod is made on a poor taper, like the Heddon 8 and 1/2 footer with a size 2 ferrule for instance. They're famous for breaking at a particular point, which is probably attributed to the taper more than anything else.