This mark creates the impression of increasing the intensity (emphasising) the letter that follows it, by leaving a small time (between a tenth and a fifth of a second long) between the end of the sound before and the sound that follows the adhak.
With explosive consonants, you just wait for the time and then start the next letter but with other consonants (such as ਲ, ਛ ਮ and so on), you don't have much choice but to start the consonant and then hold it.
Using English words, think of the stereotyped way that Welsh people say 'middle' and 'village' (with pause in the middle of the central, double letters). So, 'middle' becomes 'mid-dle' and 'village' becomes 'vil-lage' or even 'vil-le-je' demonstrating the two types of consonant.
In Punjabi, ਮੱਛੀ sounds a bit like 'Mashshshee'. If you stopped the ਛ, you would effectively turn it into a ਚ which would be wrong.
In Punjabi, using an adhak or not can change the meaning of a word . . .