Why do I need a "this" reference in a constructor but not the methods below?

If I don’t include this references in the constructor, I get the error "The assignment to variable [x] has no effect java." However, I have no problem not using one in the get method written below. Why is that?

public class Bicycle {
    
    private String brakeType;
    private int numGears;
    private int numWheels;
    
    // Constructor
    public Bicycle(String brakeType, int numGears, int numWheels) {
        this.brakeType = brakeType;
        this.numGears = numGears;
        this.numWheels = numWheels;
    }
    
    // Get methods
    public String getBrakeType() {
        return brakeType;
    }

>Solution :

To differentiate between the parameter and the instance variable