The reason to open 1CL 5/5 in a black suit is to be at level 2 if your p is weak
The hand is not strong enough to play at level of 3
Example
AJ876
QX
X
KQ10XX
I will always open 1CL with this hand .
lets say bidding go
1CL 1D
1SP 1NT
2SP>>>>>>>>>>>>>>>>>>show 5/5 or 6/5 not strong.
AQJ10X
X
KX
KQ10XX
With this hand i will open 1SP follow by 3CL