I am purchasing MacBook Pro 2017 with Touch Bar and I was told by one customer representative of the shop that you need anti virus because old one use to come with build in anti virus but not the new one.
Is it true? If so why?
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Sounds like a salesman is trying to make money off you...
Macs have never been supplied with a built-in antivirus - they do have active prevention in such as Gatekeeper but no 'true' antivirus app.
They have traditionally not been the target of most viruses - Windows viruses won't run on Macs, of course - though there are some threats out there. Nothing is truly safe from predators.
Full 'protection' is available from antivirus companies, just the same as for Windows. Some free, some paid. None of them is a complete guarantee.
Some will slow the machine down more than others, just the same as on Windows.
There is no absolute authority on "which antivirus is best" for any platform, but the closest thing to an authority might be AV-TEST.org. Until recently they only had very generic reports on Mac AV, but they have now started taking the testing more seriously.
One app they don't test [lots of politics happened, not going to go into it] is Malwarebytes, which is a free 'on demand' scanner for Mac, i.e. it doesn't run constantly, only when you tell it to. It is possible to use it in conjunction with any other live-scan antivirus.
Otherwise, never have more than one AV installed on any machine - they fight each other.
Late edit: Malwarebytes v3 is now a full on-access scanner as well as on-demand.
Many many Mac users have no antivirus at all. Macs have long been believed to be, if not immune, then "not a target".
So, the final call is yours, but don't be sold something just because the salesman wants you to spend more money.
Think it over & do the research before you put your hand in your pocket.
Since 2011 (macOS ~10.6.7), XProtect is the built in anti-malware tool from Apple.
macOS also has MRT (Malware Removal Tool) which complements XProtect in removing malware.
Ensuring this is up-to-date is as simple as checking the box to ‘Install system data files and security updates’ in System Preferences → App Store.
One thing that I want to add is that you may receive malicious files (executables, pdf files targeting buffer overflow exploits) targeted other platforms. Your system is probably safe even if you store those files. However, when you share the files (e.g. send them to others), they may harm other systems, such as those running older versions of Windows.
An effective antivirus program increases the chance of blocking the spread of such kind of files. Note that, unless your old MacBook comes with complimentary antivirus product (the usual case if you bought your MacBook from a reseller), the salesman made a false claim.
Finally, if you do want to spend money for extra security, I would suggest you buy something like a (well-known) VPN service, or a trusted hardware set (“authorised” cables, routers etc).