Seriously? 🤔
When did you start playing?
There was a very prominent myth during the first few montof legendary raids that alternating nanab and golden razzberries would "guarantee" your catch. Googling "alternating nanab" will bring up posts about and debunking it.
Nanab berries are the one berry that have a lingering effect. When you use a nanab berry, it reduces the pokemon's movement by 80%. ( The chance of using an attack, jump, or dodge, not the speed at which those animations play). After you hit the pokemon with a ball and this initial effect wears off, its movement continues to be reduced by 20% (IIRC).
One YouTuber demonstrated this lingering effect of nanabs. Another YouTuber then declared that alternating nanab and golden razzberries gave him a 100% catch rate for legendary pokemon. ...ignoring and not mentioning the fact that he was also using curballs and hitting every throw in a fairly small excellent catch circle. He implied it was some sort of bug that he was exploiting. People watching this thought they could do alternating nanab/razz part without knowing about the importance of curve balls and accuracy.
Nanab berries help you to hit the pokemon and hit within the catch circle. If you are not good at the later posted method of waiting and throwing during the end of an attack animation, then starting your first throw with a nanab could be beneficial. The two methods are kind of working against each other though. It will be longer waiting for an attack after using a nanab.