They are content on your page. They impact your site pretty much like any other content on your page.

For 99% of websites, there is no reason to allow comments.

As mentioned above, it is treated as content on your page. You should never set your blog to 'auto approve' for comments because you can safely assume that people and bots will spam your website with (mostly) useless and bot generated comments.

It is okay to allow comments but make sure to do manual reviews of comments before posting them.

Yes it can effect the "OBL" or out bound link number.

Some believe OBL effecta how powerful your ability to internally link on your page is.

It's the nature of the beast. Why wouldn't you allow comments on your blog. True, the spam bots stay busy but you have to be busier. "Real" comments show that your blog is active. Do your best to cut down on spam comments, either manually or via an automatic spam blocker, but don't ban comments.

In my opinion it defeats the purpose of blogging. You want activity.

Spam comments always increase bounce rate sometimes it's create malicious comments that can hack your site. I use Akismet and sweet captcha to protect this.

Spam comments can have disastrous effects to your websites or blog and visibility of your domain in organic search.

Of course it does, though I'm more inclined to think that they're more of an annoyance.

Most comments are spam by the way, and it will be hard to moderate them. Luckily, Wordpress plugins are there to help you filter spam.

Anyway, I still think blog comments are a way to receive traffic, and not all comments are spam. Regardless, I'd still keep an eye out.

Sorry for bad English. Automation Software commented, comments unrelated to the topic, outbound links... will reduce the quality of your site. If I had read that comment spam, I'll get out soon.

Yes. Spam comments always effects to your website. If you use wordpress site Askimet is good solution.

Yes, spam comment affect the website or blog very badly then it doesn't matter how well you optimized the site. This is the major reason to audit the site before to start optimized the site, if you found the duplicate content in your site though it is better to change the site content. Actually duplicate content affect the visibility of the site ranking in SERP.