How to use the Queen in the StarCraft 1 single player campaign

Queens are mainly casters with 4 main skills:

• It has the built in parasite spell that enables you to see the same things the host with the parasite can. (This can be removed only by the medic's restoration)
• It can Ensnare a group of units slowing them. This can be used to either prevent them from escaping or slowing their advance.
• Spawn Broodlings can also be researched to turn any ground unit into two broodlings, instantly killing the unit in the process. I don't know what mission your playing, but if that's the first mission in brood war, then you're supposed to use spawn broodlings on the ultralisks your about to come across, effectively rendering them useless. Alternately you can ensnare them and have your hunter killers have a go at them.
• Last but not least, they can infest a terran command center which turns said command center into a terrorist-training academy for suicidal infested terrans.

I don't know about the campaign, it's been ages since i played it, but Queens have several good uses:

1. They can insta-kill any ground unit with broodlings, which works especially well against defending siege tanks (sometimes causing a chain reaction destroying more of the tanks through siege tank AoE if more of his tanks are in range and fire on the broodlings), ultralisks or to harass enemy workers by turning them into broodlings which attack the other workers.
2. Ensnare is a good way to prevent enemies from retreating or delay their attacks so you have time to build up more defense. It also works wonders against enemy melee that tries to get close to your ranged units and afair it also shows you cloaked enemy units (if you manage to get ensnare on them).
3. Against CPU it's funny to infest command centers and use suicide bombers to kill anything in their path but usually there is no practical use for that.
4. The parasite is the ultimative scouting ability. Use it on as many enemies as possible (in multiplayer especially on units the other player won't select often so he won't remove it, i.e. workers or overlords) to keep track of any movement he makes. He won't be able to suprise you and afair it can even be used on enemy detectors to allow you to detect his own hidden units. For non-terrans the only way to get rid of parasites is to kill the unit in question which is especially bad on bigger units.