Is it always necessary to have to wear an expander when they have a cross bite?

Yes. When there is a crossbite, there is usually also crowding. Crowding makes oral hygiene more difficult leading to more decay and gum disease. With a crossbite there is normally also a very high palate which restricts the nasal airway and may be a factor in obstructive sleep apnea. Better to treat when young forprevention than sleep apnea when an adult.
Depends. If crossbite is dental, tooth movement will correct problem. If skeletal crossbite (upper jaw too narrow) children benefit from expansion, adults benefit fro surgically assisted expansion. All are common orthodontic treatment modalities, so ask a qualified orthodontic specialist to advise you.
Usually Beneficial . People with cross bites often close with the lower jaw off center. This makes the face appear assymetrical which is an ugly look. Palatal expansion quickly corrects all these problems permanently and in conjunction with other treatments will usually result in a dramatic improvement.. It is one of the most powerful techniques in an orthodontist's arsenal.
Crossbite. Crossbite means your upper and lower teeth are overlapping incorrectly. The upper teeth should be outside your lower teeth. Most common with your back teeth. Orthodontically, using an expander to stretch your upper arch wider is common. This can also be corrected surgically by moving the bones of the jaw but is much more complicated.