heating and holding your ingredients for lotions to help remove contamination, to reach the Krafft temperature for better emulsification, and to hit the phase inversion temperature, but there are other reasons for doing this!
Let's say I'm making a conditioner bar, like the one you see in the photo above. I can heat it until the ingredients are just melted, remove it from the double boiler, add my cool down ingredients, then pour it into moulds. But I don't have a ton of time to do this if I've just let the ingredients get to the melting point.
If I let the ingredients sit just a little longer in the double boiler, I have more play time to add my cool down ingredients, get my moulds off the shelving unit, make sure the freezer's ready to receive my product, then pour the conditioner bar. (I normally get all this ready in advance, but I'm trying to make a point here...)
lotion bars. If you take a look at this lotion bar intended for a deodorant container, you can imagine that if I just heat the ingredients to the melting point, I don't have a ton of time to pour them before they start to harden in my Pyrex jug. But if I get them to the melting point, then wait maybe another five minutes, I have much more time to play!
You don't want to go nuts and try to boil your ingredients, if you can get them to 70˚C and leave them in the double boiler to continue heating, that's just fine!
And don't get me started on lip balms! If you want to add any micas, iron oxides or other fillers to your lip balms to make them lip shimmers or lipsticks, you need to heat the ingredients beyond the melting point to ensure you have time to mix all the colours, check the colour, and pour it into your lip balm container. Definitely take this product beyond the slightly melted point to the very very melted point (I can't count how many lip balms never made it to the containers in my craft groups because the kids were having such fun with the colours, they forgot to pour them in time!)
This is one of the reasons I suggest using a double boiler instead of the microwave. You have better control over the temperature of your ingredients - you can test the temperature regularly with a candy thermometer - and you can increase or decrease the water temperature as you wish.
To summarize my ramblings - heat your ingredients a little longer than you normally would, and you're likely to see less solidifying in your Pyrex jug and have more time to get it into the intended container!