Believe it or not, the answer will frequently be "no." Technically speaking, I don't need one--my native language happens to work well internationally, but I don't need that aspect of it. Neither do many others. You probably don't need an auxlang unless you're in reasonably constant contact with people who speak another language--or more likely, several different languages.
Imagine that I'm an international businessman who knows only English. Imagine further that English is effectively unknown outside of the anglosphere. That limits me to the US (where I already live), Canada, Britain, and Australia (and several smaller spots, yes). Not a bad field. I can add Central and South America by learning a little Spanish--not even that much, really, because I probably will need a lawyer to handle contracts even in English. So I can learn some scripts for topics of interest. For anything more advanced, I can use an interpreter. I don't need an auxlang for this, because my attempt to learn and use Spanish will have an ingratiating effect that learning an auxlang won't.
Or I could be a scholar. I would need a reading knowledge of a few languages, perhaps, and if I absolutely must travel, again a few phrasebook scripts will probably do. An auxlang won't be helpful for antiquarian research such as patristics; it will only enable all my contemporary sources to be in the auxlang--and the odds are I already have more articles, studies, even mere abstracts than I can get through.
And this ignores the global reach of English.
So who does need an auxlang?
The main argument for auxlang use is efficiency: eliminate multiple translations of bureaucratic byproducts. That makes the primary target audience international organizations of whatever kind, and few people (as a percentage of the general population) are in such groups. For other people, it would be mostly helpful, not necessary. Tourists, scholars, and jet-setting businessmen are likewise small groups.
In other words, auxlangs are the province of a kind of elite--people unusually well-educated and (relatively) affluent. Classical auxlangs recognized this, which is why they leveraged knowledge of "civilized" languages such as Latin and French (and eventually, grudgingly, English). This has the valuable effect of giving them a certain commonality: if you've learned Esperanto or Ido on the one hand and Occidental or Interlingua on the other, you can probably read any western auxlang (or westlang) with little or no study.
Next I'll consider what all this means.