WordPress already has tons of form plugins such as Gravity Forms, WPForms, Contact Form 7, etc. Why would you use blocks.md over the other alternatives? Given below are some answers.
You like the approach of just writing Markdown
This is by far the biggest reason why one may pick this over the alternatives. Form builders can often be clunky and difficult to use. Here, you're just writing Markdown for everything. This can be a huge productivity boost if you already know Markdown, or take 30 minutes to learn it. After that, take a look at the examples on this website and/or docs, and you can dive right in.
You like the look and feel of the forms and pages
The forms and pages created with this plugin look exactly like what you would expect from Typeform. If you like the look, feel, and the design of the forms and pages, then this may be the right choice for you.
Pricing, Google Sheets, and other features
The blocks.md WordPress plugin is priced fairly with both tiers being one-time payments (so no subscription required). Moreover, it has some really useful features such as unlimited form responses, logic jumps, partial submissions, Google Sheets integration, data-binding, customization and branding, etc. If you want these features at a reasonable price point, then this plugin is a bargain.