Why send spies?

Numbers 13:17 says ,”And Moses sent them to spy out the land of Canaan, and said unto them, Get you up this way southward, and go up into the mountain:

13:18 And see the land, what it is, and the people that dwelleth therein, whether they be strong or weak, few or many;

13:19 And what the land is that they dwell in, whether it be good or bad; and what cities they be that they dwell in, whether in tents, or in strong holds;”  Why send spies when god made a promise in Numbers 10:9, “And if ye go to war in your land against the enemy that oppresseth you, then ye shall blow an alarm with the trumpets; and ye shall be remembered before the LORD your God, and ye shall be saved from your enemies.”