colliders and rigidbodies

Is it always necessary to have a collider AND rigidbody on an object if you simply want to know if two objects collide? I have two objects with colliders. I control the position of the objects at all time. It seems unnecessary to add rigidbodies.

I mean, what are colliders for if you always need rigidbodies to use them for collisions? I know that colliders are also used for raycasting, but is that it? Do you always need rigidbodies if you actually want to use them for collision detection?

No you don't. Rigidbodies are for easy physics simulation, by default they have gravity and movement (direction and rotation) applied to them.

So, for instance, if you added just a rigidbody to a sphere and dragged it over a cube (with just a collider attached) when you hit play the sphere will drop onto the cube, and if you tilt the cube a small amount, the shpere will roll off the cube and carry on falling. You can customise rigidbodies in the inspector to act how you want, ie turning off gravity or resricting movement/rotation on certain axis.

Colliders are just static colliders used for raycast and collision detection, they can also be used as triggers (by checking "istrigger" in the inspector) which don't have collisions (so you can move through them) but will send info depending on objects that are in/entered/exited it's trigger area. Colliders are useful for static objects such as walls and floors and triggers are useful for activating game mechanisms (ie, opening a door, playing animations, all-in-all triggering something)