Hi Phan! Welcome back!

The body of your essay is very good, but I think you could write a much stronger intro and conclusion.

Let's look at the introduction:

          Advertising and its influences on our lives have been a debatable discussion for a long time. Some people think that advertising brings us great benefits while others have an opposite opinion. Everything has two sides and advertising is not an exception, I believe that.

Many writers for IELTS use similar introductions for every topic. Let's change the introduction above to a new topic – for example, whale hunting….

          Whale hunting and its influences on our lives have been a debatable discussion for a long time. Some people think that whale hunting brings us great benefits while others have an opposite opinion. Everything has two sides and whale hunting is not an exception, I believe that.

Here's your conclusion:

Generally, in some cases, although advertising has negative impacts on our lives, it still plays an important role that cannot be replaced. On balance, it brings significant benefits to our economy and society.

and here it is about a new topic, say, air travel

Generally, in some cases, although air travel has negative impacts on our lives, it still plays an important role that cannot be replaced. On balance, it brings significant benefits to our economy and society.

These kind of introductions and conclusions are called boilerplate or cookie-cutter text and are often memorized chunks. They can really weaken your writing score in IELTS and sometimes are not even included in the word count. 

There's another example here:

Much as I have considerable sympathy with the people saying that advertising is positive to our lives, I have to mention about some disadvantages coming from this.

This sentence could be changed easily to almost any topic under the sun -- space travel, organic farming, the teaching of history in schools, the abolition of slavery, vegetarian diets, free education -- you name it.

From your clear writing ability in paragraphs 2 and 3 it's clear that you don't need to write this weak style of introduction or conclusion or to use stock phrases or sentences.

How to fix this problem…

1. Make every sentence in your introduction, conclusion, and body relevant to the topic
2. Rephrase the topic. Use synonyms, related words, different word forms
3. Give your opinion. Be blunt. Say what you mean. It's an opinion essay. You don't have to sit on the fence.
4. Give examples. Specify, specify, specify.
5. Intro: Start with a story from your experience, and then bring it back to the topic in your thesis sentence.
6. Conclusion: Finish with your opinion and something about the future

Now that the introduction and conclusion is out of the way, let's look at the body. Your paragraph two and three are excellent, especially paragraph two.

One small point: The more, the more

The more effectively advertising strategies companies use, the more customers they gain.

Make sure to keep the grammar are the word 'more' parallel (same structure, e.g. two nouns, two noun phrases, etc) 

The more effectively companies advertise, the more customers they gain. OR

The more effective the advertising strategies companies use, the more customers they gain.

 

Great work overall and I hope to see intros and conclusions as good as the body of your essays!