Existing condensate drain from the indoor units within ceiling space to gutter, Dow pipe or plumbing pipe can very likely have built up dirt/dust causing a slimy blockage in the pipe. This has the potential risk of harmful bacteria (insert better wording) for the indoor air quality.
With the slimy build up within the pipe it caused a reduction in the internal area of the pipe. This can lead to blockages and drain leaks within the roof space - leading to ceiling damage. New drains can be run with clean, clear internal pipes with added fall to drainage providing minimized chance of blockages.
If condensate drainage is deemed adequate and the installation can be re used the minimum best practice is to blow the line out with a high pressured air compressor.