If donating blood is something you like doing but don't schedule in advance, it'll be important to know when was the last time you did it, to make sure enough time has passed, but also that it didn't happen too long ago.
There are a few apps that help you track and remind you when to donate blood again, but those apps also record all that data and use it to sell you more stuff, or simply sell it to other companies, so those can sell you more stuff.
Loggit isn't like that. You own your data and only you have access to it. It's only on your device, and if you want to synchronize it across devices, you can set up your own server for it. The database synchronization engine is open source (CouchDB/PouchDB) .
It's also much simpler, and that puts you in control of what you should do with your data.