Toastmasters teaches three skillsets. By far the best known is public speaking, but evaluation and leadership are also valuable. Learning to evaluate a speech teaches you to listen carefully and to give useful feedback. The Toastmasters’ Sandwich is the best-known approach: point out several things the speaker did well, suggest some areas of improvement, and conclude with more praise.
The evaluator benefits too from the evaluation, as they hone their listening and critical skills and as they learn to give helpful feedback. The audience also benefits, as they hear both the speech and a measured response to the speech.
Outside of Toastmasters, feedback is often negative and critical ("Here’s how you’re fucking up"), which leads to demotivation ("I’m just a fuckup!"). For beginning speakers, who are often horribly afraid of speaking in public, receiving some positive reinforcement helps them realize that they already have some skills, while specific suggestions can help them improve rapidly. More advanced speakers who already know their own strengths and weaknesses often prefer the evaluator to skip a few minutes of praise and instead provide several very specific suggestions. The audience may benefit, however, from hearing the praise, when the evaluator points out things the speaker did particularly well.
A few clubs also provide a few minutes of open evaluation where members of the audience speak up and augment the original evaluation. Freely Speaking Toastmasters does this and I always find the broader set of perspectives valuable.