The facility provides the baseline. Three meals a day, a bunk, basic hygiene items for indigent inmates, and access to medical care. Nobody starves and nobody goes without the absolute minimum required by law. That much is guaranteed.
What the facility does not provide is anything beyond that minimum, and the gap between the minimum and a tolerable daily existence is where outside money makes a real difference.
Without funds on their books, an inmate cannot make phone calls. Every call costs money that comes out of their account. No money means no contact with the outside world by phone, which is one of the most isolating experiences you can have inside. Commissary is similarly off limits without funds, which means no snacks to supplement institutional meals that are nutritious but rarely satisfying, no personal hygiene items beyond what the facility issues, no extra clothing, no stationery beyond the indigent allotment.
A reasonable estimate is that the majority of inmates receive some outside financial support at some point during their sentence, though the amounts and consistency vary enormously. Some have families who send regularly. Some have occasional help. Some have nobody sending anything at all, and those inmates navigate the entire sentence on whatever the facility provides and whatever small wages they earn from a work assignment.
If you can send money, it is genuinely appreciated and it improves quality of life in ways that matter. But the advice here is important: only send what you can genuinely afford. Do not go into debt, do not use credit cards, do not sacrifice your own stability. A small amount sent consistently is worth more than a large amount sent once at significant personal cost. Your financial wellbeing on the outside matters too.