What is the use of resistors connected in series? If the current in a series circuit remains the same, then what is the use of a resistor? I read somewhere that as electrons flow through a circuit, they are accelerated towards the positive terminal, but in collision with other electrons and fixed positive ions,  they lose their kinetic energy and so their speed remains the same as they exit the resistors. 
This gave me an answer as to why the current in a circuit remains the same, but made me think that if the current is the same, what is the use of the resistors? Do resistors of greater resistances cause the electrons to collide more among themselves and lose more kinetic energy, causing the wire to heat up? If this is the case, then shouldn't the speed of the electrons decrease with higher value of resistances?
(While answering the questions, please bear in mind that I am a tenth grade student who has just been introduced to current, emf, etc. please correct me if I have made any mistake in the body of my question)
 A: Well, the thing is the concept of flow of current is very easy to misunderstand. As you are the tenth grader, you must be struggling by the definition of current as rate of flow of charge i.e. $$ I = \frac{q}{t}$$.
The concept of flow is used because the fluid mechanics was the immediate precursor of Electricity .
Now, coming to your logic of why the same current flows through all the resistors when they are in series, I don’t think (personally) that that’s the correct logic. Same current flows through them is an experimental fact, and it’s explanation can be illustrated in the light of fluid dynamics. Here (https://www.quora.com/Why-does-the-current-always-remain-the-same-in-a-series-circuit-When-the-charges-encounter-a-resistor-wouldnt-the-speed-of-flow-decrease-If-the-speed-decreases-shouldnt-the-rate-of-flow-decrease-at-the-same-time) I have given a link to a very nice explanation of the concept which you have mentioned.
Now, your main question 

What is the use of resistors connected in series?

Suppose you have connected a very high resistance resistor (around 10,000 ohms) directly into your home plug which creates a voltage of $220 V$ , then the current flowing through our resistor is $$ I = \frac{V}{R}$$
$$ I = 0.022 A$$ and now you come with some coiled tungsten contained in a glass and have two terminals ( I mean a bulb) and connected it in series with our high resistance resistor, then $$ Power = I^2 R $$
$$ Power = 4.84\times 10^{-4} ~R$$
Well you see, just by increasing the resistance you can get as much glow as you want ( Power) and resistance can be increased just by taking a longer tungsten wire ($ R = (length /Area) \times \rho) .
And again, it’s natural fact that same current flows through the resistors when they are in series and it’s use is something we have to find, it’s all on us. It’s not something invented for a purpose but was found to be true and hence use depends on how we use it.  
Hope you have found it helpful.
