not sure what you mean by 'track your clients'? If by that you mean support them, then you shouldn't need it, since you should be able to reply to mails, etc..
The only benefit I see to either (android or ios) app is the ability to setup and approve new accounts, but maybe I'm missing something there.
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If you have a fairly recent (1-2 years) Android smartphone then logging into WHMCS using the browser or Chrome would do just fine. I've tried the mobile app and did not like it at all, it felt like it was missing a lot of features. It might be hard to navigate around WHMCS without a "mobile layout" but it does the job just fine.