Walkie-talkies are robust, easy-to-use, and simple (with relatively few parts and features),
so they're brilliant for use outdoors and great for children (or for children who need to stay in touch with parents, for example, on vacation).
They're particularly handy in places where cellphone network coverage is poor or unavailable (in disasters or emergencies, for example).
They're also great fun: kids love doing the "over" and "over and out" business.
They're very handy in situations where lots of people need to listen and only one needs to talk at once (for example, if you're giving instructions to workers on a camp site).
They're extremely convenient, weighing just 100–200g (3.5–7 oz); work over a decent range (typically 5–10 square km or 2–4 square miles); and have long battery life (20 hours on 3–4 rechargeable or alkaline batteries is typical).
Walkie-talkies generally have multiple channels (from about 8 to 25 or more) so you can easily switch to another frequency if other people are also using walkie-talkies nearby.