The reasons why testing by developers is very important

Early detected defects are easy to correct. In general, the cost of fixing defects will rise in time.(Boehm, 1981)

High quality basic elements make it easier to establish a high quality system. Low quality basic elements, on the other hand, will lead to an unreliable system and this can't be solved practically by functional tests.

Defects detected during post development stages are difficult to trace back to source.

Defects detected during post development stages have to be corrected and this will lead to time consuming regression tests.

Good testing during the development stage has a positive influence on the total project time.

Straight testing of exception handling is only only possible at unit level, where execeptions can be triggered individually.

(Bart Broekman and Edwin Notenboom, "Testing Embedded Software")