Is there any advantage to recording walks?

Recorded activities will be used when determining your acute and chronic load along with your recovery. It's also used to compute your lactate threshold and estimated 5k, marathon etc completion times.

These are pretty important if you're someone who wants to maintain a certain load when training for an event or even trying to maximise fitness.

If you're not running / riding (and recording those) and you're just looking to track your steps, don't post your workouts to Strava, don't review your workouts or don't know what load ratios are then no, theres no point.

The Fenix is absolute overkill if you're just looking at tracking your step count and sleep.

I trail run and hike extensively 

okay so the walks still contribute to the load and such - but does such a low-level casual walk actually affect that?

Kaitlyn said:

I know garmin has some feature to auto-detect activity?

There are some watches from the "Sport and Fitness" series (e.g. the Venu watches) that have an activity auto-start function.

However, the fenix series does not.

However, the fenix does have the "Move-IQ" function. This function recognizes certain movement patterns and displays them on your daily timeline. However, this is not an automatic start of an activity profile.
The "Move IQ" function is explained here:

https://support.garmin.com/en-US/?faq=zgFpy8MShkArqAxCug5wC6 

Kaitlyn said:

I trail run and hike extensively 

okay so the walks still contribute to the load and such - but does such a low-level casual walk actually affect that?

Very, very marginally. I'm in the same camp as you and I'll sometimes record walks just to get the Z2 time in order to maintain a balanced load focus (saves me from having to do a whole bunch of Z2 runs in order to convince the watch). Note that if you're pretty for (which it sounds like you are), that'll need to be a very brisk walk to crack into Z2.

Short walks I do not record, I see no reason to. Longer walks I do record, because over time I can see how my body (based on HR and pace) behaves. After period of inactivity in workouts, my HR is usually higher and pace lower during walks, but after some time of exercising, HR becomes lower and pace faster even though I do not try to affect those numbers, I just walk comfortably. So, there is no actual advantage, other than it being interesting for you observing you. 

Longer walks, especially hikes, do impact recovery metrics.