At the present levels of technology it would be very difficult to for mankind to explore beyond the immediate region around Earth.
There has to be a way of reducing the costs. Establishing colonies on other planets and moons would be a practical way to do it.
Colonisation of the planets will facilitate future exploration of our Solar System
The Moon may have valuable resources we do not know for sure. It is speculated that Helium 3, an element that is possibly useful for nuclear fusion, is present. Water has been found as ice trapped in dark corners of the Moon that are never exposed to sunlight. The only way we can really find out what is there is to do some serious prospecting. That would require machines, men to maintain and operate the machine, and a base to house the men and machines. It is unlikely that mining will be economical but the Moon is one sixth the gravity of Earth and so offers a very cheap alternative for producing spacecraft from locally sourced materials and launching them at a fraction of what it would cost from Earth. There is a good argument for constructing a base on the Moon.
Mars also offers the potential for using locally sourced materials to support the further exploration of space. Mars is also smaller than Earth so it does not cost as much to launch spacecraft. There are plans to land people on Mars in the near future. Mars is long way away so the trip would not be short. When they get there the astronauts would require some form of habitation to provide a base for exploration. Old lava tubes have been proposed as a means to build a base cheaply and they are underground so also have the added benefit of protecting the astronauts from radiation that gets through Mars' thin atmosphere.