make file trying to figure out postgresql source code DESTDIR mean

make append and separate by white space
https://superuser.com/questions/360178/what-does-make-install-do
Trying to understand the following make command.

installincludes:
    $(INSTALL) -d '$(DESTDIR)$(includedir_server)/'
    $(INSTALL_DATA) $(addprefix $(srcdir)/, $(RELATIVE_INCLUDES)) '$(DESTDIR)$(includedir_server)/'

rg is rigrep regex command, like grep.
To understand this, I first begin with includedir_server.

 pg_config | rg server

return

INCLUDEDIR-SERVER = /usr/include/postgresql/15/server

In folder /src (postgresql source code) do

cat Makefile.global.in | rg --line-number 'includedir_server'

return

170:includedir_server = $(pkgincludedir)/server
248:override CPPFLAGS := -I$(includedir_server) -I$(includedir_internal) $(CPPFLAGS)

To understand $(INSTALL) -d '$(DESTDIR)$(includedir_server)/' now I feel like the follow logic is true.

$(includedir_server)  ==  $(pkgincludedir)/server  == /usr/include/postgresql/15/server

But if the above logic is true, then $(includedir_server) is already a full path. Then what is $(DESTDIR)? also $(INSTALL) -d what does this part do?