Does celery powder have any non-preservative use?
Yes, it tastes of celery. Celery is one of the three ingredients of mirepoix, the vegetable mix which is ubiquitous in French cuisine and has spread to many others. Current large-scale food production rarely includes slicing fresh vegetables into small cubes and browning them in a pan, but they try to add the ingredients in more convenient form. So, if you eat powdered soup or similar, it can be used for that. 

On the other hand, if you are eating cured meat, then the most probable reason are indeed the nitrates. If you are looking at a certain type of cured meat (e.g. bacon), it is impossible to produce the same cured meat without nitrates *, but there are customers who are scared of "chemicals" and are more likely to buy the food if the label states that the nitrates are coming from a plant source. 

* And in general, most types of curing are done with nitrates. There are types done without nitrates, e.g. the prosciutto pointed out in the comments, but they are the minority. And it is more difficult to create a safe process which cures without nitrates, since they have a preservative role.