Different PyCharm version when downloading from within IDE

why doesn’t Jetbrains offer me to download 2021.2.3 within the IDE?

Installations are normally cumulative. For example, the installation of PyCharm 2021.3 is made on top of the previous version 2021.2 not 2021.1 – see Other Versions.

Inside PyCharm you are offered the updates in steps instead of updating straight to the latest version. This can be convenient for a number of reasons:

You may want to try the previous version for a time before going straight for the latest (if you are on a team and have a policy of using the same version the other team members are using).
The downloads are smaller taken in separate versions (some users are on limited bandwidths and may prefer to do the updates/downloads in steps).
Sometimes PyCharm will discontinue support for older Python versions – see Supported languages. In that case you may want to update an intermediate version but not to the latest. So updating from within the IDE gives you that option for convenience.

what would I loose if I upgrade only to 2021.1.3 instead of 2021.2.3?

You would loose two things:

Bug fixes.
New features.

The new features that come with each version may, or may not, interest you depending for what you use PyCharm. But the general user interface tends to improve with each version, see What’s New in PyCharm 2021.2.

For a more detailed view of changes and specific bug fixes between releases see PyCharm 2021.2 Release Notes.