When ya need it, ya need it.
If ya don't need it, ya don't need it.
(But all soil always needs more compost.)
Insufficiently-amended clay, or sand with little organic content and little mineral content: these need fertilizer.
(It also needs the organic matter in compost, but you can haul and add a few pounds of NPK easier and faster than cubic yards of compost.)
If you have enough raw materials and space and energy to make plenty of compost for all your beds: you probably won't need extra fertilizer after a few years of amending the soil.
In my case, my compost heap at it's largest was tiny, and I have to haul bags from Home Depot in my trunk, because the CG bulk compost I've seen is mostly sawdust: expensive sawdust!
I tend to expand my beds faster than I can acquire compost. So, for me, fertilizer is a stop-gap. I'll need it until I find sources of cheap, good compost and mulch that haul themselves into my backyard and spread themsleves around.
More likely, I'll eventually run out of space to make more raised beds where there's any sun. Then, one bag of compost at a time, I'll bring all my clay up to being pretty fair soil.
And hopefully I'll be able to haul enough compost each year to keep the soil healthy and fertile despite the organic matter decomposing every year.
In the absence of abundant compost, it makes the difference between slow, stunted, short, scrawny, struggling, pathetic seedlings succumbing to slugs, vs. vigorous healthy plants.
