Today, many of us do the bulk of our writing on our phones, while our PC usage is getting more and more restricted to work. You might sit down on your laptop to send an email to your boss, but you're probably typing a lot more text through the day in the form of text messages, social media updates, and even less formal emails.
And even though displays get bigger, typing on a touchscreen can often leave something to be desired. To speed up the process as much as possible, we all take shortcuts, so things like capitalisation and punctuation often take a back seat to sending a message quickly. Spellcheck can protect us not from misspelled words and messages like "c u 2nite", but errors of capitalisation, article usage, prepositions and the rest are unfortunately way too common, and not caught by spellcheck.