The function of the "ifneq ($(MAKECMDGOALS),clean)" part in the Makefile - makefile

Question

I really don't know the function of the following part:
ifneq ($(MAKECMDGOALS),clean)
-include $(DFILES)
endif
Here is the possible explanation I get from the GNU make manual:
to avoid including ‘.d’ files during clean rules, so make won’t create
them only to immediately remove them again:
But I don't fully understand "won’t create them only to immediately remove them again".
Here is the Makefile from derivative.tar.bz2 from http://www.dirac.org/linux/gdb/03-Initialization,_Listing,_And_Running.php#wherearewegoingtogo:
TARGET = driver
# CC = colorgcc
CC = gcc
CFILES = $(wildcard *.c)
OFILES = $(patsubst %.c, %.o, $(CFILES))
DFILES = $(patsubst %.c, .deps/%.d, $(CFILES))
WARN = -W -Wall -Wstrict-prototypes -Wmissing-prototypes -Waggregate-return \
-Wpointer-arith -Wcast-qual -Wcast-align -Wmissing-declarations -pedantic \
-Wnested-externs -Wredundant-decls -Wwrite-strings -Winline -Werror
CFLAGS = -std=c99 $(WARN) -g3
LDLIBS = -lm
all: $(TARGET)
ctags *.c *.h
$(TARGET): $(OFILES)
$(CC) -o $(TARGET) $(OFILES) $(LDLIBS)
.deps/%.d: %.c
#mkdir -p .deps
#$(CC) -MM $(CPPFLAGS) $< > $#.$$$$; \
sed 's,\($*\)\.o[ :]*,\1.o $# : ,g' < $#.$$$$ > $#; $(RM) -rf $#.$$$$
ifneq ($(MAKECMDGOALS),clean)
-include $(DFILES)
endif
.PHONY: clean nuke
clean:
$(RM) -rf $(TARGET) *.o core .deps tags

Answer 1

Let's suppose you've just untarred the archive, and for whatever reason you want to run make clean before anything else. Presumably, it is already clean. Now, without the ifneq, make would:
Execute the include $(DFILES) line. Before doing the include proper, it would...
Run the recipe for .deps/%.d: %.c because the $(DFILES) variable contains files with names that match .deps/%.d. This means running $(CC) (which is gcc by default), which is a rather expensive operation. Once the recipe is run as many times as there are .d files to generate, then...
Run the recipe for clean.
All the work done in 2 is pointless since at step 3 it will be deleted. The reason 2 exists is that when you include a file, make first checks whether it has a recipe to generate the file to be included an runs the recipe if the file does not exist or is out of date.
The ifneq bit allows the Makefile to avoid doing the work in step 2 if the goal is clean.