Yes they are good. However if you have plants in a conservatory, they will contribute to condensation on glass and walls if you don't have adequate condensation. Dehumidifiers by their nature dry out the air, dry the potting compost and some plants don't like this.
Damp has several causes:
Condensation from plants and cooking and lack of ventilation
Water getting in under tiles and slates through damaged felt and soaking down through plaster at the top of the wall
Moisture soaking through solid concrete, stone or brick walls or through cracks in render on walls. Modern brick or block walls are double layered with a cavity in between. The cavity is filled with insulation. This prevents heat and damp bridging the gap. Dampness however can soak in around window sills if the sills don't have a means of causing water to drip off the sill (e.g. a slot on the underside)
Damp can rise through brick or block work if a damp proof course isn't installed. This is simply a layer of PVC which acts as a moisture barrier and inserted between courses of bricks