The border is 1,988 miles long. There aren’t that many natural barriers that would mean only a few hundred miles of wall. I suspect you don’t live near the border.
Actually, I used to live in South Texas.
Large sections of the border are nearly impassable desert and a long way from any town or road. The wall is most needed in stretches where there are cities, towns, and roads that make crossing easier. Probably hundreds of illegal aliens already die every year of dehydration trying to enter the US illegally. If they had to walk another 75 or 100 miles that number would be in the thousands or tens of thousands. That would represent a huge disincentive to crossing the border.
Several hundred miles of wall, in the right spot, would make a huge difference. It would probably do 90% of the job at far less cost than the Gulf of Mexico to the Pacific ocean wall, especially when combined with other measures. That’s how I understood President Trump’s promise. If he can build a wall across the whole border great, but if he gets 400 - 500 miles built, in the right place, it will be more than enough for him to claim he kept his promise.