As far as I believe the idea behind HODLER it to take step forward and bring cryptocurrency payments to everyday life. If you only use hardware wallet as a storage that is fine. HODLER has got a vision where you can go to any shop and pay securely for your groceries with cryptocurrency of your choice. This would finally allow us to live without the influence of greedy bankers and central banks. Bringing the power back to normal people.
Following that an additional functionality is needed, without compromising security. HODLER is a hardware wallet meaning it is offline most of the time. Only about 1 sec connection is needed when you are sending payment. Your phone on the other hand, is online 24/7 and you have got many apps installed on it, each being a potential threat to your cryptoassets. Many of us are guilty of trying games apps or playing Quizzes on Facebook or similar. All of them require to accept T&C's which no one bothers to read. This is the main reason why your wallet needs to be separate/independent from your phone, or even computer.
I have written an article explaining the gaps in security of hardware wallets that HODLER team addressed: https://steemit.com/cryptocurrency/@alesa/do-you-think-your-hardware-wallet-is-safe
More detailed description of the security features of HODLER can be found on the website and specially in this document: https://hodler.tech/docs/SecurityDescription.pdf
RE: Cryptocurrency payment terminal for high street retailers - "Retailer Mode" in HODLER Wallet