Omega 3 fatty acid α-linolenic acid (ALA) is considered essential unlike EPA and DHA since the latter two can be synthesised from it. Suppose one gets their EPA and DHA from oily fish, will they no longer need ALA i.e. does ALA alone have any important/essential roles for the body or is it only there to be converted to EHA and DHA and then only these two are used by the body to carry out important functions?
I think, at this point, the answer to the question: "does ALA alone have any important/essential roles" is not yet known, but here are few hints:
The main role of ALA was assumed to be as a precursor to the longer-chain n-3 PUFA, EPA and DHA...ALA accumulates in specific sites in the body of mammals (carcass, adipose, and skin).... There is some evidence that ALA may be involved with skin and fur function. There is continuing debate regarding whether ALA has actions of its own in relation to the cardiovascular system and neural function.
ALA does have importance beyond being converted to DHA and EPA, although it is found to be less efficacious. See this study:
The conversion of ALA to EPA lies between 8-12%, while it's conversion to DHA is less than 1%. ALA is either eliminated from the body by metabolism in the liver (eliminated as in chemically altered to be excreted. stored as a monounsaturated fatty acid, or utilized as energy) or stored (without being altered) in cell membranes.
The Budwig Protocol is an interesting theory about ALA storage in cell membrane and it's application through diet.
I hope this helps.
[EDIT] To better answer your question, according to the study I linked above, you would not need ALA if you are able to eat adequate amounts of DHA and EPA. ALA's most unique function (according to Budwig protocol, which is still speculative but very intuitive) is it's desired health effects when it accumulates in cell membranes.