The advantage is that they don't get a bunch of huge attachments.
But over the years I've come to the realization that attaching photos is still the best way to share photos with grandparents.
Scenario 1 (attach to email)
Grandpa receives an email with an automatically generated thumbnail and clicks the thumbnail to see the larger version.
Scenario 2 (everything else)
Grandpa receives an email with a link. The thumbnails probably don't show up because images are turned off by default and that's how Grandpa rolls. Grandpa clicks the link which opens a new tab and shows similar thumbnails to scenario 1. Grandpa clicks to see a larger version.
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Granted, you can provide a bunch of additional value (like ordering prints and whatnot) in scenario 2. But it comes at a very steep cost.
Disclaimer, I'm a founder of OpenPhoto and work about 30 yards from the Kicksend team. I personally use their app and think they're on to something. But I'd be lying if I didn't say it's a challenging road (for them and us).
I haven't used the app yet, I'm about to, but what's the advantage over just emailing the photos to the grandparents?