Since water doesn't really compress under pressure, and copper pipes are pretty rigid, any heating of that water will cause it to expand and instantly raise the pressure. The parts that might help contain that are the supply hoses to faucets, washing machines, toilets, etc. that can start to balloon (stressing them and shortening their lives in the process). But those are all designed for domestic water pressure, and that is limited to 80psi or lower. The T&P valve is designed to open above 150psi (or 210-degrees). You risk blowing out a weak hose, as well as causing a seal to leak on any valve. If all of the valves and hoses are in good working condition, the pressure will spike almost instantly while heating and you stop using any water anywhere (any small leak can relieve the pressure). Ideally, especially since the T&P valve on the WH is designed for emergency use, you'd prevent it from opening on a regular basis which could cause it to either never close or not open and allow things to get way out of hand. So, if you have a properly sized and installed expansion tank in your system, the bladder in that tank can compress the air on the other side, allowing the water to expand and if the thing is sized properly, the pressure will barely register as rising at all. Once you open a faucet, the water that is pushed into that tank will come out, putting the bladder back at its neutral position, ready for the next cycle. They do wear out, they do need to be eventually replaced, but they can last a significant number of years and they're not expensive.