To clarify, you can link it whether you pay for the subscription or not I believe, but there will only be one log & it’s primary function will simply be calculating so there won’t be much to see.
The subscription allows you to keep unlimited logs and set up multiple pools. For instance I have 3 “pools” set up. My pool, my hot tub, & “pretend pool” which I use to help calculate things for others here without changing my settings and having to change them back later.
Keeping logs allows you to then view summaries of various parameters and additions (like how much fc or cya have I added this month or this year etc.)
Along with the time line records of results, additions, notes, maintenance etc. which is what we mean when we say logs.
I like being able to scroll back to see when I opened or closed last year and what my parameters were at certain times of the season etc.
poolmath is also the only app I have ever paid for. I used it without a subscription for years and just kept a notebook but I would always seem to forget something and it would be confusing when I looked back on it. When I decided to get a larger pool I got the subscription about 7 years ago or so. I haven’t regretted it.