Yes- wasted energy in electricity or gas. If leaking from the case of the unit itself it generally means the cylinder inside has ruptured (new unit required.). If leaking out of a valve this means that the valve may need replacing.
By all means, but the cause of a dripping tap is 90% of the time due to the seat needing to be re-machined, which requires a tool that most people do not have. Very rarely is it just the tap washer that needs replacing. *Please be aware that if your house / unit floods as a result of repairs to plumbing carried out by an unlicensed person, the insurance company may decline insurance claim*
Sometimes the ball float may just need adjusting. Every toilet cistern has an inlet valve and a outlet valve. Both have washers and rubbers in them that do need replacing. I generally show regular customers how to replace them and leave a few washers for them to use if they show some interest in doing the job themselves.
Not really. Prevention is better then cure- grease should not be put down the pipe. Products like Drano are effective in de-greasing but should be used with EXTREMECAUTION. I’ve known people to be blinded by the highly-reactive Sodium hydroxide component of de-greasers.
Hire companies have drainage machines for hire, however if you are using the “rigid” brand of drainage machine, be warned thay are extremely powerful and for the inexperienced weekend warrior, they can just about pull your hand off if your hand gets caught in the cable when it twists up from hitting a snag down the pipe (eg tree roots).