What is a term to describe something that is not necessary to begin with, but becomes so as a result of a non necessary choice?
One of the senses of entailment applies:
entailment
...
- something involved as a necessary part or consequence of something:
Long hours of work are an entailment of the job.
[Dictionary.com]
That (originally unnecessary) thing has now become necessary because it is incidental to the other activity (choice that you made).
ODO:
incidental ADJECTIVE
2 (incidental to) Happening as a result of (an activity):
‘the ordinary risks incidental to a fireman's job’
‘For instance, a charity has to refrain from political advocacy, unless such lobbying activity is merely incidental to the charitable purpose.’
If thing A is not an inherent requirement for an overall process P, but is a requirement if you choose to implement thing B, then A is a requisite for B.
A requisite is something required to achieve a desired goal or comply with something else. For example, if you work at Joe's pizza emporium, you can make pies, but if you want to deliver the pizzas, you need a driver's license. Having a valid driver's license is a direct requisite for being a Joe's delivery driver. Furthermore, having a valid license requires having a certain level of car insurance, so having that insurance is an indirect requisite for delivering Joe's pizzas.
A possible future condition, indirect effect or result of something (e.g. a choice) is an implication.
Definition: something implied; a close connection.
Example: The newspapers discussed the implications of the president's election.