Webb will have significantly larger field of view than the NICMOS camera on Hubble and better spatial resolution than is available with the infrared Spitzer telescope.
Hubble's extreme deep field image below. Some of those faint patches of light in it are galaxies dating back more than 13.2 billion years, Webb is expected to look even further to observe pioneer galaxies and stars. Ore we discover the universe is even bigger than we know.
To look closer in our own galaxy, there could be 300 million potentially habitable planets to discover.
So much new space porn coming, can't wait for it.
... this telescope is especially interesting because it will be stationed at one of the Earth-Sun Lagrange points out beyond the moon, L2 Lagrange point to be precise, so essentially in a solar orbit like the Earth itself https://webb.nasa.gov/content/about/orbit.html
... several sci-fi themes are that the best place for an Galactic Empire type organisation to monitor Earth from would be on the moon or at one of these Lagrange points, since we have essentially inhabited earth orbits now with satellites and the space station ...