Ok. I understand you don't know what it's like to be evacuated, and why people need supplies.
I had to leave my home way back in 1997 and actually we lost our home to the Red River flood back then, so I kinda know a bit about what it was like but we had much more time to prepare.
First off, in Houston, the flooding is so bad, thousands lost everything. EVERYTHING. i mean house, clothes, furniture, appliances, toiletries, pets, lamps, books, beds n blankets, tv's, cars, trucks, campers, outdoor equipment, etc...
Those from up north had to be flown out in a hurry and couldn't bring very much at all.
Ok. Why need donations?
Grab some of your clothes, and clothes for your kids/spouse if you have and get in your car as if you were going to the park or for gas at a gas station.
Drive to a completely different city where you don't know anyone and pretend to be an evacuee. You have to find affordable shelter, pay for meals, basically depending on circumstances, start your life over again.
Many people can't afford to do that. Babies need to eat, poop n be safe and warm. Imagine you only have $300 in your account and you just got evacuated somewhere and you might have or did just loose your house, the stores you used to shop at, etc...
Ok. Now new city. $300 to buy groceries, find shelter, new job, clothes if you need, toiletries, rent etc...
This is where good volunteers, helpful people and the Red Cross come in, finding and providing shelter and food, formula and diapers, blankets and beds, hope and happiness for those displaced from their homes. Homes they may never see again. That way you can try keep some of your cash if you have any to help rebuild your life later... Your going to need it!
If you still don't get it, go for a drive... A very long one and just try rebuild your life from out of a backpack. Possible, but better with help. Happy motoring