SMS can be slow, as normal calls take priority over sending text messages on a network (which mean that they come first). There is also a limit of 160 characters that a text message can send. Longer messages than this will be sent as multiple messages, where the sender has to pay for each individual message.
Text messages are often sent between phone users when an actual phone call is not wanted or needed. For example, they may be used in a situation where silence is needed, or to send something private that one person does not want people near them to hear. Message price depends on the phone network of the user. Some mobile phone plans allow for unlimited texting.