Guest posts on website - advice

A friend who owns an SME business, keeps getting 3-4 emails a day asking about making a guest post on their website. The emails generally consist of the following:

Example:
"I understand that you would be the person to talk to regarding getting a potential guest post on your blog. Would you mind letting me know the different options available?".

Another example:
"I’m a new reader of your site and I went through some of the blogs posted and I think they are fantastic!
I’m writing to you because I’d love to contribute a guest/sponsor post to your domain.
I’ve been brainstorming some topics that I think your readers would get a ton of value from, so if you are interested, let me know and I`ll send some over to you".

I'm not sure if its relevant, but 90% of the emails that make contact are from a Gmail email address (not URL/business domain address)
What would they benefit from this? Is this just a way for the individual to promote their product from a company that ranks well in a particular sector? Are there any benefits to a business owner using a guest post.

thanks in advance