Honestly, it depends on the bench style and where you are installing it. If you have a standard surface-mount street bench with pre-drilled feet and you are anchoring it to a flat concrete pad or paved area, it is usually a very manageable DIY job. You will need a hammer drill, a masonry bit, concrete anchors or wedge bolts, a socket wrench, a level, and a second person to help position the bench. Most people can complete this type of install in about 30 to 90 minutes.
The basic process is simple: mark the mounting holes, drill into the concrete, clean out the dust, line up the bench, and tighten the bolts. The biggest challenges are the weight of the bench and keeping the holes aligned. If you take your time and use a level, it is very doable.
However, if the bench is an in-ground model with legs or posts that need to be set into concrete footings, the project becomes more involved. You will need to dig holes, possibly below the frost line, mix and pour concrete, and brace the bench while it cures. A confident DIYer can do this, but it is physically demanding and less forgiving if something shifts.
You should probably hire a contractor if the bench is very heavy or custom, the site is sloped or unstable, you are installing near utility lines, the project requires permits, or the bench must meet public accessibility codes. For commercial streetscapes or multiple benches, a contractor is often worth the extra cost.
So for most standard benches on concrete or pavement, you likely do not need a contractor. Just grab a friend, the right masonry drill bit, and some patience. For in-ground installation or public and commercial projects, calling a licensed professional is the safer choice.