Hello @Ewelina_nina
Welcome to the forum!!
That is a very good question. I suspect it is because a due time could ave an impact on a follow up task but a start time is left for the individual to decide. I do see however some use for this feature.
While asana does not offer that, why not give Clockwise integration a go. It is newly released and it is integrated with your calendar so it helps you more with your schedule management and when to work on things. So this might be a good solution.
Here is a forum post about it from @Jason_Woods
And here is the blog post about the integration that asana launched:
Hope this helps!
Thanks,
Rashad