It's ridiculous and only serves to tie you to Automattic's crap. The only true purpose I can find is that it allows them to use Jetpack to track your site's traffic. 90% of JP is bloatware full of shit you can't disable and don't need. Lets go through them:
WordPress Stats: It's powered by Quantcast, so it isn't something that WP developed on their own. It's just a limited skinning of data you can already get by adding Quantcast's tracking code to your site.
Comments: WordPress already has built-in commenting, why the need for a secondary system?
Subscriptions: Not necessary for the vast majority of people or sites. You want people to visit your site, don't you?
Contact Form: Most themes include a contact form template and there are plenty of plugins out there that don't require you to log into someone else's site to use them.
Sharing: Most themes include social media and there are tons of plugins out there that don't require you to log into someone else's site to use them. BTW, isn't Enhanced Distribution (below) the same thing as Sharing?
Gravatar Hovercard: One of the most useless pieces of shit on the planet and virtually impossible to remove from WordPress. I have a profile photo for virtually every social network, why do I need to create yet another one on another site that only works on other WordPress sites? 90% of readers don't have gravatar accounts, so the fugly icon is used instead. Thanks Automattic.
Shortcode Embeds: WordPress and themes already does this.
WP.me url shortner: Most social media sharing plugins already do this (and wordpress has a built-in shortner already). Why do we need this?
Beautiful Math: Really? To whom is this a need?
Extra sidebar widgets: If this is so important, why not build it directly into the next version of wordpress? There's also already plugins that do this.
Enhanced Distro: Sharing plugins do this. WordPress SEO does this. BTW, isn't Sharing (above) the same thing as enhanced distribution?
Custom CSS: If someone's changing their theme's css, do they really need a plugin to allow them to do it? If they know how to code CSS, they're doing it in a child theme or directly into the theme using ftp.