Impact of Terminator Efficiency in Genetic Constructs
To add to gaspanic's answer, not all terminators work equally well under all conditions. So if terminators weren't characterized in the organism or tissue you are working in the efficiency might be different. Additionally, though rarely people might be looking to design with a leaky terminator mimicking bacterial use of leaky terminators to use one DNA sequence to create two mRNAs of different length.
[Edit] Sources for further reading
Example of terminator efficacy differences between different species
Example of terminator differences between tissues
Terminator Attenuation - stopping of termination only sometimes
Leaky stop codons as a molecular tool