Has Donald Trump laid out a plan for this wall or ever talked about it with more detail to clarify what his ultimate plan and goal may be to build a wall?
Trump is generally light on details. However, the wall is one of the stronger positions he has (in terms of describing a plan to accomplish it).
He plans to tax/confiscate money sent from the United States (US) to Mexico. The idea being that such money is often from an undocumented immigrant to family remaining in Mexico. This is intended to have two effects. First, it pays for the wall. Second, it makes illegal immigration less attractive, as one of the drivers is providing for families.
Note that there is already a plan to build the wall. The wall would consist of two barriers, one for vehicles and one for people, which would each cover 652 miles (the length of the land border not already impassible).
Can anyone think of any reason why a wall would be good otherwise even though a tunnel dug beneath it is a way though?
Mostly because digging a tunnel takes time. If the border patrol notices while the tunnel is being dug, they can stop it. If the border patrol finds a tunnel after it is finished, they can fill it in.
It's also possible to build walls deeper as well as taller. Digging a tunnel through clay is one thing. Through bedrock is another.
Drug dealers may be unwilling to share their tunnels with illegal immigrants who may share the locations of the tunnels for preferential treatment from immigration officials.
Remember that part of the point here is simply to make illegal crossings unprofitable. If someone has to pay for a $10,000 or $100,000 tunnel to be built to cross, that eats into other profits. Maybe it's cheaper to apply for documentation. Or buy legal marijuana in California.
Another issue is to stop walk throughs. A hundred people or more die each year while trying to cross the border. If the crossing were more difficult, fewer people would try. Note that these people don't have advanced tools like ladders or shovels. They have enough trouble carrying enough water.