Grammarly is on a roll. The popular auto-editing software tool has raised $200 million from Baillie Gifford and other investors at a $13 billion valuation.  Grammarly is not a particularly exciting company. It helps delete unnecessary prepositional phrases and other grammatical errors — not exactly earth-shattering technology. But as in-person communication becomes less common and written communication — be it Slack messages, texts or email — grows in use, Grammarly’s value proposition seems solid.