Nobody can! LOL. It's kinda like when your parents tell you to eat your spinach and you dont want to.. you ask why? they say its good for you and trust me... and you say but......and they say just do it!!
Here's the best explanation I can think of. Say your flashing a custom rom and you decide to flash a 6.1 rom (sys 21055), then you want to make a change and you flash another custom rom which is 6.5.1 (23504). You are going from one completely different sys to another, a carrier specific to generic (let's suppose), and different software combinations, versions of youtube, opera, bing, keyboards, etc, etc. Also you are bringing files that were saved in device memory (yeah some things seem to stick in there) and especially internal storage that are not completely compatible so you start getting minor errors like youtube failures and tf errors. You end up having multiple email cache saved to internal and album cache which could lead to problems. Or more important stuff like data is not working or you soft reset your phone and it hard resets. These are the common problems people experience and the
only way to avoid them, is the to flash the shipped rom of your carrier which will reset your phone back to it's factory settings, then you can start from a clean slate. Some will disagree that you need to do one more hard reset post flash, which I don't do to be honest, but the main thing to do is flash the shipped rom. A good tip would be to go through your internal storage and remove folders like youtube that are saved from previous flashes. I usually go through internal storage>progam folder and remove folders that are saved that are unnecessary. for example if Bing is installed on the custom rom, then you are not going to save it to internal so remove it from internal storage>program folder.
Having said that if you are using NRG's rom series and he is staying in the same sys and its a minor update, you probably are good to flash away to the new rom. I would still review the internal storage in this case before flashing to the new rom. ok this was too long...