Whenever a piston engine doesn't have a camshaft in the cylinder heads, it is usually located far from the valves and rocker arms, which means it needs pushrods to transfer the camshaft's movements to the valves.
In the example pictured at the right, the lower intake manifold was removed to reveal the pushrod channel, where you can see the push rods leading up to the rocker arms, which are attached to yellow valve springs. The engine's single cam can barely be seen running below the metal engine block support down the middle, and the camshaft timing gear (with the timing chain wrapped around it) can be seen at the very bottom of the photo.
Having only one camshaft, replacing or upgrading the camshaft is less expensive than multi-cam engines. It also reduces the amount of timing chains and gears used, versus a two-head (V6, V8, etc.) engine. Another benefit is overall engine size, since the heads and valve covers can be shorter due to the reduced space which would otherwise be demanded by an over-head cam design.
Pushrods are a weak point in the valve system, being relatively thin, long rods with a lot of force to deal with. Putting variable valve timing on a pushrod engine is so difficult there have been few attempts to do it, and the design has more moving parts than an engine with cams that simply push down on the valves from above them. Replacing a camshaft in a pushrod engine often requires the engine to be at least partially removed from the vehicle before it the cam can be pulled out.