hi Francis,
think of the crystal reports viewer being used like the adobe pdf viewer. most people consume / use pdf's compared to the number of people that design them.
not many people really need the crystal reports designer, only those who create or modify the actual .rpt file.
with the crystal reports viewer you can have end users view reports in .rpt format which has more or different functionality than looking at a report which has been exported to another format such as word, or pdf. things like drill down, saved data prompts (to filter out records, change groups, etc.)
if you've got businessobjects enterprise or crystal reports server you can then schedule or publish reports to end users using email and then they can open up these .rpt files without having to touch the server product or have additional server licenses.
i hope that this helps,
jamie