To prevent any back water from the shower head and also the water contamination into system.
More "VOLUME" flowing from the tub spout then the shower head as the shower head has a flow restrictor
To conserve water.
Any maximum flow shower head will give maximum effect with minimum water usage.
Did you remove the shower head and check for a flow restrictor or mineral deposits blocking the orifices
4 gallons if head delivers 2 gallons/minute
Standard high-flow aka unrestricted shower heads made before 1995 were water-guzzlers. As of 1995 shower heads must be low-flow, with a rate of 2.5 gallons per minute.
An average flow rate cannot be determined as there are many different shower heads and each house in every country has different water pressure and not everyone may use full pressure when using a shower. But, approximately said that the average flow rate of shower water 2.5 gallons per minute at a water pressure of 80 pounds per square inch.
to much water flowing through shower valve will cause whistle in pipes.try a new shower head with less flow[gallons per minute]amount should be on head somewhere try 2.5 gpm or less
A dirty or clogged screen can cause the low water presser remove shower head and clean. If not it could be calcium deposits built up on shower head if so soak shower head in a cleaning solution for several hours if that dose not work buy a new shower head. (note some shower heads have a low flow re-stricter in some cases you can remove this and get more water pressure)
About 1.25 gallons per minute.
depending on your spout diverter if its a valve or is it on the spout it self, if the diverter is on the spout then your diverter is not letting full flow of water out of the spout and its putting enough pressure to allow the water out of the shower head, I had this problem before and if you change the spout it should fix the problem. If you have a 3 way valve which is on the wall then i would suggest replaceing the parts in that valve if possible other wise change out the divert valve. If your broke like me and want a cheap fix then put either a water flow resistor or a shutoff right before your shower head.