How important are grades/GPA for obtaining a job in this field?

You are asking: How important are grades/GPA for obtaining a job in this field?. I'm going to assume you are referring to the business field and that you are applying to companies through your college's on-campus recruiting.

What you are learning in school is a very basic foundation of what you will use through your business career. The better you learn the basics of business the more quickly you will be able to grow and add value to your future job.

Grades are an indicator of how well you have acquired and internalized the basic knowledge and skills that you will use at your job.
When businesses are looking to hire students, they will receive many more resumes than they have positions to fill. Businesses want to hire the best without spending too much time reading and processing every single resume.

As a result, business may decide to begin the resume selection process by only looking at resumes that have GPA’s higher than a certain threshold such as 3.0 or a 3.5. Within that smaller group resumes they will read deeper for interesting and compelling experience.
Companies will often leave a few interview spots for interesting candidates that connect well with the recruiters at the on-campus events (meet-and-greets, career fairs, etc.). Help the recruiters know that you are a candidate that brings experience, perspectives and capabilities at a higher level than your GPA suggests (such as experience building and managing organizations at school or relevant work experience).

tldr: a good GPA is the gold star on your resume that help people want to read it.

Hi, Abbas! Thanks for your question. In the fields indicated by your hashtags - and, I suspect, in most fields - grades/GPA are an important consideration because they tell the hiring manager (1) how well you have mastered the basic materials presented to you and/or (2) how hard you worked to master them. All else being equal, a candidate with a significantly higher GPA may have a better understanding of business fundamentals and may "catch on" more quickly to the demands of the job. But it is very rare that all else is equal, and I am generally willing to consider candidates with a somewhat-lower GPA if I see something on their resume that indicates drive and ability beyond academics. People learn in different ways and universities usually teach in only one way, so the fact that someone did not excel in academic studies does not mean they will not excel in the actual field of work. Conversely, someone with a 4.0+ GPA may flounder in real-world applications. It's hard to know from reading a resume - which is often one among dozens or even hundreds received for a single job opening - what the GPA alone really means, but it's definitely a factor to consider.

If your GPA is not particularly high, make sure there's something in your resume or cover letter that catches my attention and makes your grades a smaller component of the total you. Tell me about leadership skills you've gained and demonstrated beyond the classwork. Show me your success in practical applications - in part-time jobs, volunteer work, military duty, etc. If the hiring manager is fixated on whether you received high marks from a top-tier business school, the other info may not matter. But here's to hoping the person receiving your resume is smarter than that.

how much of a difference does a good or bad GPA make when applying for a job?

Grades/GPA may likely be a factor when competing for your first job opportunity out of school however employers are looking for well rounded employees and will consider additionally your record of accomplishments in the workplace, internships, volunteering, etc. Going forward from there, your future potential employers will be more focused in on your skills, and how well they match the requirements of the job/role; also looking for solid record of accomplishments versus other candidates