You may be thinking, as I was, that after this segwit thing got activated that infamous August 1st deadline was irrelevant.
On August 1st, another group will be launching a new coin called Bitcoin Cash (BCC).
While you may or may not like BCC, it currently has (at the time of this writing) a futures value of $ 553 (20% of BTC)
When BCC launches on August 1st, it is being launched as a "fork" – meaning the private keys that exist for BTC will exist with BCC. So if you have 10 BTC in a wallet in BTC, you will also have 10 BTC in a wallet for BCC.
Now it may seem like free money, but nobody knows whether on August 1st the combined sum of BCC and BTC will equal what BTC is now – or whether it will be more/less. Whatever happens, it is likely the price will be volatile through first few days of August.
Now, why move out of an exchange like Coinbase? Exchanges own those private keys, and can decide whether their users will also own money in this new "fork". Coinbase has announced that they are NOT planning on support BCC.
Of course, this could change – Coinbase and other exchanges, seeing that BCC has a lot of value, and not wanting to piss off customers who didn't realize/understand what is going on, could be "nice" – but they (arguably, rightfully so) don't want to always have to support every fork that some group wants to make from each currency they support.
If you have only been using Coinbase and don't know how to set a private wallet up, here's a link with different ones you can use.
TLDR; If you do not move your money to a wallet you control, you WILL (unless BCC goes to absolute $ 0 after Aug 1st) lose money.
*EDIT – I just wrote a step by step guide on how to do this with Electrum; hopefully this helps someone!