Why bother having different engine options on one aircraft? When an airline purchases a new passenger jet, they often have a choice of engines (from different manufacturers). Why is this?

I would expect that one of the engine options is "best" and that all customers would buy that option. Thus it wouldn't be worthwhile to certify and offer the other (unwanted) options. Evidently this is not the case, so the question is why.

I can think of two possible reasons for this:


Different customers may have different requirements (e.g. shorthaul versus longhaul routes, operating in hot or cold environments) and different engine options have differing performance (e.g. better cruise efficiency versus climb efficiency or better performance in hot/cold temperatures). But I can't imagine that the requirements would be significantly different and, if they were very different, they would buy a totally different plane, not just a different engine.
Business/legal considerations: Perhaps airlines have contracts with engine manufacturers separate from aircraft manufacturers. e.g. British Airways may have a special relationship with Rolls Royce (possibly due to trade tariffs). But, again, I would imagine that airlines would always seek the most efficient engines, rather than signing such contracts.


Are either (or both) of these reasons right or is there a totally different reason?
When Lockheed built their L-1011 TriStar, they had one engine option, the Rolls-Royce RB211. RR did not deliver in time, and that has been attributed to the airplane's demise.

Although the TriStar's design schedule closely followed that of its competitor, Douglas beat Lockheed to market by a year due to delays in powerplant development.

So, having more than one engine option lowers the risk of a new airplane launch, and this has the benefit of allowing the airlines to pick their favorite.
Having one engine option on the other hand, like with the Boeing 777-300ER and the 777-200LR/F, allows cheaper certification, but it frustrates the customers:

Even if the 777X programme is successful, the sole-source deal has left some customers frustrated. Boeing argues that it chose the GE90 as the best engine for the 777X programme after its own rigorous competition. But as one airline executive says: &quot;Our response is we'd rather run our own competition.&quot;
He adds the suspicion that Boeing decided to take the benefits of the engine choice competition for itself rather than let it accrue to the customer. Also, by eliminating a choice of engines, the company saved the costs of certifying more than one of them, he points out.

Back to your points: Your final conclusion is right, the performance difference is not a factor. For the second point, airlines get maintenance deals, so they like to stick with the manufacturer they already have business with. Getting discounts is easier when there is competition.

Related and interesting: How much would it cost for a large airline to switch engine manufacturer for their planes?