Health Connect is a platform consisting of a set of APIs where multiple apps and services can read (or write) your health metrics. Right now, fitness apps must manually integrate with other similar services to share data. For example, if you use Strava to track your runs, other apps like Sleep My Cycle can't see the data. Similarly, if you use Sleep as Android for monitoring your sleep quality, other fitness tracking apps can't view its data if the app does not add support for them.
Health Connect aims to solve this problem. Every fitness app can write its collected data to Health Connect, which another app can pull. This way, apps can share their data with each other without any privacy risk.
You decide if you want to enable the sync, and also on a per-app basis, as well as per type of health metric.
Many health apps on Android support Health Connect syncing, such as Samsung Health, Fitbit, Oura Ring, etc. I, for example, have a Fitbit Aria scale that measures my weight, and with Health Connect I can have that metric synced to my other health apps (it also depends on whether an app pushes only to Health Connect, or whether it also pulls in data.
Samsung Health is a better health app than Google Fit, but if you are planning on changing in future from a Samsung phone to say a Xiaomi phone, then you're going top lose all your data inside Samsung Health. If you had set up syncing via Health Connect to a 3rd party health app that will also work on Xiaomi (eg. Google Fit) then you'll be able to access that health data after you have migrated phones.
See
How to sync Samsung Health data with Google Fit#
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interoperability 
Finally, all your health and fitness data in one spot!
