[structural mechanics] the reason why the influence line under joint load is different from that under direct load

General books are about

The method of the influence line of the main beam support reaction and internal force under the node bearing mode ：
• Bearing reaction 𝐹𝑅𝐴 and 𝐹𝑅𝐵 The influence line of , Same as above .
• 𝑀𝐶 The influence line of ,C A point is just a node , Same as above .
• 𝑀𝐾 The influence line of ： The following describes its practice first , And prove it .
1. Make a section K A certain amount of Z Influence line under direct moving load , and
Determine the vertical mark corresponding to each node .
2. Connect a straight line between two nodes , Then we get the
Influence line .

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Why node connections , I think it's very simple , It is because the shear force at the two special joints will suddenly change , And then because the shear force is constant , So the bending moment changes linearly , So the bending moment between these two special joints is linear , We also know the value of the two points , Then as long as a line is connected, the linear condition and the endpoint condition are satisfied

But he still didn't explain why the two loads
My idea is

But further , The analytic formula that changes at two special nodes does not want to be calculated , Because we have to set Fp Relative distance between and section , Just think about it , lazy