I was casually chatting with a dev manager who had worked in the trenches for 25 years. He emphasized how a lot of developers spend their career without leaving a legacy. You work on a dozen systems, for different companies, and a decade later don't have anything external to show for it. Sure, you've got skills, 10-linear feet of obsolete tech books you've read, and the memories. But it's not as tangible as shipping actual products (like the devs who can say "I helped ship AoE2 - that was me!").
This hit home with me as I reach the 5-year mark for my blog. After 5 years of consistent blogging, I've written 430 posts and received hundreds of comments (many of them educating to me). There are a lot of obvious benefits to blogging, but after writing for years, what really sticks out to me is the legacy of blogging. I started blogging at CSC, blogged all through Paylocity, and continue blogging now at CareerEd. Ironically, my blog topics have evolved from hard-core development, to tech lead, to architecture, to more managerial-related tasks.
I can't show people any of the systems I've done (except for the occasional screenshot from a dusty tech doc), but as appropriate, I can share with them the archive of hundreds of blog posts. It helps motivate me to want to write for another 5 years.