Adapters adapt between different connectors that are electronically compatible, allowing different types of interfaces.
You might also need to convert interfaces from one format to another - for example Ethernet to USB.
DVI-D / HDMI
These two are electrically compatible, and since they are both digital interfaces, they do not require signal conversion nor have a loss of signal quality.
DVI-A / VGA
Likewise these two are analogue connections and thus are compatible. However, only 640x480 resolution is officially supported.
VGA to DVI-D requires a converter to convert analogue to digital signals.
USB / Ethernet
USB to Ethernet requires converters. This is particularly common with USB-C to USB-A to Ethernet chains on newer laptops.
USB Hubs
These allow several interfaces into a single USB connection