Scales are the very basis of almost all piano and keyboard playing. While it is true that anyone can learn a piece of music without knowing any of their scales, for example using popular methods like YouTube, Yousician and other modern learning platforms, if someone wishes to become an actual pianist and take it seriously then scales are the basis of everything. Scales are important for all of these below - this list is not exhaustive:
Dexterity - being able to move your hands quickly and fluently around the keyboard
Constructing and understanding chords - again anyone can play chords without knowing scales BUT they will not necessarily know what they're doing
Relative pitch and being able to hear and understand music without an instrument present
Composing music - with intention
Just the above list covers so many fundamentals of playing the piano and keyboard. For example, dexterity allows the playing of melodies and complex broken chord arrangements, understanding chords is important when accompanying on piano/keyboard especially in contemporary pop and jazz. Yet there are many more things that knowing your scales are crucial for as a musician.
Scales are like the alphabet of music - the very basics and foundation of successful musicianship.
You should as a bare minimum, learn the following scales:
The links below will take you to sites where you can learn how to play these scales on piano
Piano Exercises - Technical Exercises
Learning your scales is just one part of the numerous technical exercises and fundamentals you should know
Technical exercises are crucial to master the piano/keyboard. Think of a footballer. They don't just play games but instead they actually train more than they perform. A premier league footballer would usually play just once or twice a week. They would train a minimum of four or five days out of that week. The same applies to musicians - you should train more than you perform, and certainly train with exercises more than you play songs that you enjoy.
Below is a link to 'The Top 5 Piano Exercises for Beginners'
https://youtu.be/tQh3-WBzaKY
Below is a FULL theory lesson - 3 hours long! Contains everything from the basics of what a piano is to constructing scales and the circle of fifths:
https://www.youtube.com/watch?v=Ud9CpGOG1GE
(YouTube channel below). An excellent piano resource site for advanced improvisation licks and tips as well as introduction videos for scale work and improvisation:
https://www.pianopig.com/
https://www.youtube.com/channel/UC-obvt6Gn0Eef9vKxwcg_pg (YouTube channel)
This video explains the major scale - https://www.youtube.com/watch?v=pF4DegadNYs&t=164s
This video explains HOW you should practise your scales - https://www.youtube.com/watch?v=ETuU6_86KaI
Another excellent free online site with free lessons and lots of clear and useful tutorials and advice
Piano exercises for beginners - https://www.youtube.com/watch?v=g4G8VQ194sc&t=275s