If you plan to apply for aviation roles and are exploring whether to become ground staff or cabin crew you'll know you need training when you realise you lack the required skills or formal certification that airlines demand. According to the article How Are Ground Staff and Cabin Crew Jobs Different both ground staff and cabin crew must undergo training before starting their job.
You don't have training in airport operations customer service check in systems baggage handling all required for ground staff.
You haven't learnt safety procedures first aid emergency evacuations in flight service grooming and other airline specific protocols essential for cabin crew.
You don't meet the fitness grooming or physical requirements (especially for cabin crew).
You want to be eligible for selection by airlines most airlines expect a recognised certification/training from a reputed institute for both roles.
If you join a proper training course here's what you'll learn depending on your choice:
For ground staff: airport operations check in & ticketing systems baggage handling documentation soft skills and passenger interaction at terminals.
For cabin crew: safety protocols emergency/first aid in flight service grooming in air customer service cabin procedures and more.
You should look for training before applying for the job ideally when you decide you want to pursue career as ground-staff or cabin crew. If you have none of the above skills or certifications enrolling in a recognised aviation training course is necessary.