Returning to university after my industrial year, I took a very dim view of the academic environment and resented being asked to do this task that was worth no credit towards my degree.
So I didn't rehearse or even make any slides, I just stood up and talked about what I had been up to.
And although I was by any measure an extremely inexperienced speaker, it was the best talk I had ever given. It was the first time I stood in front of a room of people and felt present in the environment while giving my talk, rather than monotonously reciting the rehearsed material.
So obviously different people have different experiences, but I learnt that day that rehearsing your talk isn't always helpful.
It helped that I really enjoyed my industrial year and had loads of interesting stuff to talk about. So maybe the more important thing is to be interested in the topic.
I was amazed at how naturally and well he did. All he wrote down were 6-7 topics to talk about. He got a huge applause.
All this is not great for the audience (who have "invested" into your session, by paying for the ticket, spending time away from work and family, not attending other concurrent sessions, etc.), and it can so easily be avoided by rehearsing.
The most common reason I have seen for folks skipping to rehearse is the awkward feeling you might have when speaking loud all by yourself. If that's the issue, it can help to do a dry run in front of colleagues. In any case, "winging it" is best reserved for later on, after having gathered quite a bit of speaking experience and having spoken about the same, or very similar, topics before.
I'd also recommend to avoid reading from slides during a talk as much as possible, it's also not a great experience for the audience. There shouldn't be much text on slides to begin with, as folks will either read that, or listen to what you say, but typically have a hard time doing both at once.
(All this is a general recommendation, not a comment on your talks which I have not seen)