Every time you open Rstudio or R you should see something like:
R version 4.1.0 (2021-05-18) -- "Camp Pontanezen"
As you can see it specifies my version and a specific date. That becomes a default "point" from which R will try and install packages and try and assist. To keep compiling and keep things reproducible as far as possible there are some default snapshots in place - essentially sets of packages with their respective versions etc that was available at certain times based on the version of R and that date. If this wasn't the case then you could have had serious issues of installing incompatible packages with newer or older versions of R which would lead to extremely complex library management and compiling problems.
The other reason why snapshots are so invaluable is that it gives us a frozen point in time at which we can point and keep things reproducible. When I am programming on a project that ends up in production I can guarantee that everyone who will have to install my library or project will have the identical set of packages with underlying code as I specified the snapshot and thus the respective versions.
If you ever want to get an idea of the snapshot at your disposal I suggest looking here: Home MRAN is not entirely CRAN but based on it similarly and you can specify a version and date and then see which packages with versions were available on that point.
Introduction to renv • renv renv does a lot of this too and you even have the function
renv::snapshot() you can look at.
For a lot of people they never have to worry about the above but it is always important to keep in mind when you intend to share your code etc.