In the first case, the first statement is hypothetical (using the conditional mood), so considers a hypothetical situation where the machines would need to work, and weren't going to work without energy.
The other is a basic true statement (using the future tense) that simply states that the machines won't work without energy. I should note that this statement doesn't mean they can't work without energy, only that they won't. This is more to do with the idea of free will: consider a person who is able to work without biscuits but refuses to.
The second case is a lot more fun. Saying a machine can't (or cannot) work without energy sort of states that the machine couldn't work without energy even if it wanted to. In this case, because machines aren't sentient, it is identical to saying they won't work without energy.
However, the first sentence in the second group is either conditional or imperfect. In the case where it is conditional, it talks about a hypothetical situation like before. If, however, the tense is imperfect, it implies that the machines, in the past, weren't able to work without energy, but now supposedly can.
Note 1: Machines need energy to work. Fact of physics.
Note 2: I have no way of knowing what tense 'couldn't' is in. Normally you'd need to use the context to work it out.
One possible explanation about his joke could relate to sales technique and the use of biased questions. If you were a sales person in a shop, you would be expected to walk up to customers and offer to help. If you say:
Can I help you?
This is not a biased question: the customer can answer yes or no. If, instead, you ask
How can I help you?
this is a biased question: you are assuming that you can help, and it's difficult for the customer to refuse. This question is simply a polite biased question: you could go to the next level by asking a very obsequious biased question:
How can I be of service?
When your friend asked you to help, your response was perfectly appropriate because you definitely know that your friend wants your help. It is, however, a stock phrase which sounds like a sales pitch, and your friend may have been alluding to the "sales pitch" angle by suggesting that you should go to the next level.
If somebody asked me to help them, I would reply slightly less formally and I would avoid potentially confusing stock phrases, by saying:
Sure. What would you like me to do?
Best Answer
To answer this question, I first need to explain an interesting quirk of the phrase 'be supposed to'.
When used in the present tense, as in the example given by the Merriam-Webster dictionary, the meaning of 'be supposed to' is positive.
However, when used in the past tense the meaning of 'be supposed to' becomes negative.
As stated in one of the comments above, the original examples contain some logical problems, so I'll answer this two ways.
If the main point is the attendance to the concert:
In the first case, you intended to go to the concert but did not go because it was raining.
In the second case, you went to the concert. It is implied with the negative conjunction 'but' and following phrase that you did not enjoy the event as much as you might have because it was raining.
Alternatively, if the emphasis is the mode of transportation:
In the first case, the original plan (to take the bus), the rationale for the change (it was raining), and the final course of action (I decided to go by car) are all clearly stated. 'Instead' is not required for this sentence, but as a native speaker I find it almost impossible to say this sentence without attaching the word 'instead' at the end.
In the second case, the final course of action (so I decided to go by car) implies a change of plans, but the original plan is unknown (e.g. bus, train?).
In the third case, without the final course of action, two potential meanings are possible:
Generally this shortened sentence without the 'final course of action' would only be used if key facts such as 'the concert was cancelled' or 'you drove to the concert' was already known from earlier context.