Hi,
Both type Custom and Retrieved are used when we want to fill in the table via code (could be your qaction, could even be another element / script).

The main difference is that custom does not support multiple cell sets for that column, where retrieved does.
This is a historical thing: we first had type Custom to fill in data with methods that would set 1 cell at the time. (e.g. set parameter by key)
Later we improved and supported methods to set multiple cells for that column (e.g. fill array)

Today, best practice is to simply use retrieved every time the table is filled by code. This way we allow multiple cells (different rows) for that column to be filled in via 1 method.

Personally I only use type Custom when the column is a button. This way it is visually clear for me (when looking at the table xml definition) what is supposed to be filled in (column params of type Read) and what is a button (column param of type write). This is just a personal preference. using retrieved for write params will also work, it just does not make any sense to have logic to fill it with data (since it cannot hold any)

Hope this clears things up.

1) Are there specific use cases / benefits of custom columns over retrieved columns?
See above. I would always go for retrieved to allow multiple sets (different rows) for this column.

2) Will changing a custom column to a retrieved column cause any major change or impact I should be aware of?

No, using a method that sets 1 cell in a column that supports multiple sets will still work.