Warehouses gives you room for (illegal) goods. Storing goods require maintenance, which costs money and staff.
Having at least one warehouse gives you access to automated logistics.
Logistics
You can start scheduling routes and this will automatically start generating jobs for you Criminal organization to pick up.
You do NOT need a warehouse for this. This can be already achieved with the safe house computers.
Automated logistics
When this is unlocked you can now hire drivers and start automating your logistics which create a different type of transport job for you Criminal organization.