Why is neccessary a LO 10 MHz external reference?

I'm design an LO module. I've designed a first version using a internal clock (an integrated of 10Mhz).
But when I've looked for commercial LO, I realised that all has an external reference of 10 Mhz. Is the reason that an external reference in much better, so the phase noise is improved?

Thanks!

The main reason is for frequency stability, generally using a GPS synchronized signal generator.
The phase noise is not improved.

What type of undesired consequences can I have with a bad frequency stability?

Thanks again

Depends on your system! Cell phone base stations use time of arrival to figure out position, and the actual time/frequency is important for each parallel receiver. Other applications, it does not matter at all.

The reason for a 10 MHz is largely historic, 10 MHz crystals have an extremely good temperature coefficient. So in the 50's kit that needed an external reference used 10 MHz for standardisation. Now if you get a rubidium standard, its output is , guess what?, 10MHz!
Frank

In the telecomm systems, the standard of freq stability is always about 5ppm(5e-6). That includes temperature variation, short period stabilty and long term stability, even for years.
Using external TCXO 10M usually have about 2ppm stability.