OK, so you have your trailer tongue weight (or a rough idea what it is). What next? If you have a weight distribution hitch that uses spring arms, it's very likely that you will need to raise both the front of the trailer and the rear of your tow vehicle in order to be able to get the spring arms onto their brackets. So not only is the tongue jack lifting the weight of the front of your trailer, it is also picking up the rear of your tow vehicle. 😳
If a trailer didn't have a trailer jack, this means of transport would be pretty much useless – unless you are strong enough to lift it by hands or hitch (unhitch) a tow vehicle somehow else. Jacks can be manual and electric. With the help of an electric jack, you can hitch up substantially easier than with the manual unit, since a manual jack requires the application of physical force. Anyway, whatever the trailer jack you have, this part has to be completely workable and sufficiently reliable to prevent troubles during a journey under various conditions.