You are welcome, be seated, be comfortable
Ushers assist those who may need help finding a seat, directions to the washrooms, answer any questions they may have or direct them to who can answer their questions.
The ushers help bring the gifts up during Mass and let the Special Ministers of the Eucharist know who needs communion brought to them.
They also take up the collection and gifts during the Mass. If someone gets ill or needs medical care, they do what is needed in those circumstances.