Capital accumulation
$a=0$ means implies the household does not enjoy leisure. In other words, the trade-off between consumption and leisure is switched off. It's not generally true that with $a=0$, utility is higher for a given value of $c_t$, as this depends on the previous value of $l$. Whether or not consumption rises as a consequence depends also on the parameter $b$, i.e. the utility discount factor. 
If the household does not value leisure, he might work 80 hours per week and obtain a higher income. If he is very forward-looking however, he might also save a lot for later consumption.