Is the letter "f" necessary if the decimal point is written?

Not necessary, but best to use f-less constants with double variables/math and f suffixed constants with float ones.

Aren't both float?

There are both floating-point.
But float and double are different.

Type difference

Appending an f make a floating point constant type float rather than double.

This can steer to different code with _Generic and <tgmath.h> as well as the math used in a floating-point expression.

Given the usual range and precision difference, subtle and sometimes not so subtle differences will occur.

Value difference

The f tell the compiler to use the usually narrower range and precision of a float.
e.g. 0.1 != 0.1f.

Sometimes it makes a value difference too, even when assigned to a float.

Floating-point constants with an f or F suffix have type float, with no suffix have type double, and with an l or L suffix have type long double.

First, this makes a difference because the value can be different, and usually will be different if the value is not an integer. For example, using the formats most commonly used for float and double, the value of 123456789. is 123,456,789, but the value of 123456789.f is 123,456,792.

Further, if the value is assigned to a float, the result may be different than if the constant used f originally. For example, after:

float f0 = 9007198986305535.5f;
float f1 = 9007198986305535.5;

The value of f0 is 9,007,198,717,870,080, but the value of f1 is 9,007,199,254,740,992. This is because the former rounds 9,007,198,986,305,535.5 directly to float, but the latter rounds first to double and then to float.

The f suffix makes the constant 10000.0f have type float.

The constant 10000.0 has type double.