All the above, plus filing old pages gives you something to look back over, I still have the diary page for the day I met my now-husband, the little notes I made, especially the ones in the days before and immediately afterwards bring back some amazing memories... that wouldn't happen if it was a digital entry in a calendar.
But they're also faster to take notes (grab, open, scribble down thoughts, put away again) than doing the same on a phone, the batteries never die and they don't need upgrading every year or so, they can't be hacked, and older models don't become unusable or un-synchable, I have some inserts in mine I've had for over 6 years, and some of the accessories (clear wallets for photos, rulers and dividers) for over 12!
By contrast, my first ever mobile phone is an unusuable plastic brick by now (adaptor and battery both dead, and replacements never worked) so all my texts and stuff are locked away, never to be seen again... whereas I have diary pages on paper dating back to teenage angst.
There are some great planner-community blogs where people share their ideas, "Philofaxy" is a good jumping-off point, and there are a few threads on here, it's one of the things I went looking for after I joined because I'm passionate about using a GTD ("Get Things Done" by David Allen) system in my Filofax and wondered if that was shared on here. Filofaxes and other makes are the ideal "SLG-meets-practicality" item IMO!!