I've designed quite complicated boards without a schematic. However I have also made mistakes in some quite complicated boards. Now I will always try to have a schematic, even if it is drawn after a lot of the layout is complete. The PCB software can check that a layout and a schematic agree with each other, and it is a lot easier to check on a schematic that wires go where required.
There are some extra constraints that need to be followed when getting the software to compare the schematic with the layout. In many circuits that are connections or gates that can be swapped, and PCB software like DesignSpark won't accept the swaps, so extra work is needed to get the checks to pass.
Examples of swaps that usually make no difference are:-
Which way round passive components are fitted.
Which inputs of simple gates go where, like dual-input NAND gates, or inputs of bridge rectifiers.
Which gate or amplifier or whatever in a multi-gate package is used.
On microprocessors, many pins have multiple functions, and can be interchanged and sorted in software.
Also the schematic may not show you anything about physical layout, so you need to separately check rules like keeping decoupling capacitor close to ICs, keep ground wires short and clearances where high voltages are used,