wxClientDC is primarily useful for obtaining information about the window from outside EVT_PAINT() handler.
Typical use of this class is to obtain the extent of some text string in order to allocate enough size for a window, e.g.
Note: While wxClientDC may also be used for drawing on the client area of a window from outside an EVT_PAINT() handler in some ports, this does not work on all platforms (neither wxOSX nor wxGTK with GTK 3 Wayland backend support this, so drawing using wxClientDC simply doesn't have any effect there) and the only portable way of drawing is via wxPaintDC. To redraw a small part of the window, use wxWindow:refreshRect/3 to invalidate just this part and check wxWindow:getUpdateRegion/1 in the paint event handler to redraw this part only.
A wxClientDC object is initialized to use the same font and colours as the window it is associated with.
wxWidgets docs: wxClientDC