It's not a value used for design, for design you mainly used Young modulus (rigidity) and yield strenght (mechanical limit before irreversible deformation).
Elongation at break (A%) represents material ability to permanently deform without breaking and is a metric of material brittleness.
For exemple Glass has a high yield strenght but a very low A%, and if you drop it it breaks.
You dont want you 316l part to break this way for safety reasons, if you somehow overpass yield strengh the part will deform (a lot for 316l, around 40% of initial size !) before breaking, adding some margin to the design.
Usually in additive manufacturing, manufacturers presents UTS, yield strenght and A% (and sometimes Young modulus).
And people using thoses machines are tracking these 3 values to evaluate part quality, when A% drops it usually reveals process defects ( porosities, fissuration etc..)
ie EOS :
ie SLM solution :
