In creo , Why we should use weak dimension in sketch model & what is the use of shift & alt button ?
Basically you use "weak dimensions" because it's less time-consuming and maybe less complex.
In order to create "strong dimensions" you use the relations tool which is quite time-consuming because you need to select the dimension and add a number (e.g. s18=40) and complex if your sketch has many dimensions.
On the other hand, designing with "stong dimensions" provides you with the safety that your sketch will not change its shape and dimensions.
I hope this helps!