It's more a matter of if you cannot return...for instance overcast at 300' and vis of 2000m..ish
...only available approach has a minima of 400'/1200m (CNS Rwy 15 ILS)...well you dont look likely to get back in for an emergency return but just XXnm away is an airfield where weather is comfortably above Cat 1 minimas...or, for that matter CAVOK...and you determine that in that direction there is no terrain that precludes you getting there and you have the fuel to get there on one engine...that's called a departure alternate and you can legally depart at 300/2km.
No such option then you cannot depart, in the above case, with less than 400'/1200m..or whatever it happens to be where you're at.
Another example might be departing YSBK and holding YSSY as a departure alternate...it gets you away when you might not otherwise.
Chuckles