Dependencies necessary for developing java web project

I just started to make myself get used to develop java web project with maven in IDEA. However, I found out those basic dependencies like servelt or apachetaglibs should be implemented manually in pom.xml, which were unnecessary in normal java web project. So what's the reason?

By the side, i really eager to know which dependencies are really indispensable. It already takes me pretty of time for figure those issues caused by the dependence.