If you are going to have expensive fish in there the cost of a UV may be worth it. Most people wouldn't consider them a necessity. High quality food and water will prevent 95% of the problems you would ever encounter.
What they kill is based on flow rate and UVC bulb wattage. For a given wattage lower flow will kill more things. Higher flow will just kill algae. Lower flow will kill bacteria. Lowest flow will kill parasites, bacteria, and algae.
You can run one independently on its own powerhead or have it plumbed in to your filtration system depending on how your filtration is plumbed.