Learn French: Nous avons voulu éviter la file.

In the passé composé, the verb vouloir often shifts in meaning from "wanted to" to "tried to" or "decided to." Because the passé composé focuses on completed, concrete actions rather than ongoing states of mind, nous avons voulu implies that you actively took a step to avoid the line, rather than just having the desire to do so.

How would the meaning change if I used the imparfait and said Nous voulions éviter la file?

Using the imparfait (nous voulions) describes an ongoing state of mind in the past, so it translates simply as "We wanted to avoid the line." It tells us what your intention was, but leaves it completely open as to whether you actually did anything to avoid it, or whether you just ended up stuck in the line anyway.

Why is there no preposition like à or de between voulu and éviter?

Vouloir is a modal verb, much like pouvoir (can/to be able to) and devoir (must/to have to). When you follow a modal verb with another action, it connects directly to the infinitive form of that verb without any preposition in between. Therefore, you just use vouloir + infinitive.

I normally use la queue to say "the line." Can I use that here instead of la file?