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Signing a document to show your agreement is very common, but signatures seem very easy to forge. Nowadays, with electronic signatures it is trivial - some software doesn't even include your actual signature but pastes in your name in a cursive font.
What good does it do to sign something, then? Couldn't you just claim you didn't sign anything afterwards?
The issues you describe have existed with signatures from the beginning of their use. There exists a tradeoff between ease of use and reliability, both of forgery and of people falsely claiming forgery.
Originally, the closest thing to a signature was the use of seals and signet rings. While relatively hard to forge, it only showed that the possessor of the object agreed.
Signatures, especially in cursive font, were developed later. They were in some ways easier to forge(you didn't need to get access to a physical device), but more difficult in others(the seal symbols tended to be used on everything and various improvements in technology had been made), and harder to falsely claim forgery(because most people can't alter their handwriting well). You were affixing your name to the document, indicating that you agreed. Often, the signatures were required to backed up with the signatures of other people as witnesses. They didn't have to agree to the document, they just had to agree to testify that you signed of your own free will.
Because witnesses, especially trustworthy and independent witnesses, are hard to come by, some places have dropped that requirement, such as checks and signing a aper receipt when using a credit card. But for some important documents, certain jurisdictions still require witnesses, including large transactions (a document relating to a car insurance payout I recently had required a witness to confirm my signature) and marriages.
However, with electronic media, the point of a signature is more to indicate deliberate acceptance of terms, with verification of an individual being left to other processes (e.g. IP address, MAC address, linkage to a specific email account, etc.), so forgery is less of an issue.
I have also seen "signatures" amount to checkboxes and "I agree" buttons. Generally, the higher the stakes and "more legal" the agreement, the more likely to these have been the "typed signatures" that you describe, but this seems to be decreasing in frequency, suggesting that its purpose was to stop gap a hole in legal acceptance by judges/courts/laws with regards to electronic communications.
Addendum: It should also note that the replacement of seals by signatures is not universal; for instance in Japan, seals are still used over signatures in the majority of cases.
What good does it do to sign something, then?
A signature still tends to reflect a meeting of minds despite the greater ease of forgery through (ab-)use of technology.
Couldn't you just claim you didn't sign anything afterwards?
Yes, but the parties' conduct and other circumstantial evidence can outweigh or disprove a party's after-the-fact allegations.
Among other factors, a person's denial that he signed a document is significantly less credible where the document at issue pertains to a neutral non-party.
For instance, the lawyer who represented the defendant in one of my cases made the fraudulent allegation that I did not serve upon him a copy of my motion (and of course, with that pretext he asked the court to be awarded with $1,500 from my pocket). Seven days later, he changed his version, now alleging that I served the papers on the defendant (instead of on him). Since I sent the papers by Certified Mail, the lawyer's fraudulent allegations imply his false denial that he signed a USPS receipt form.
I disproved that crook by filing in court a copy of the USPS records with his signature (see last page of my Exhibit, thereby showing that this lawyer himself received from the USPS the papers I sent by Certified Mail.
This lawyer-crook kept denying the issue in court. However, since the USPS is a non-party and has no interests vested in the lawsuit I filed, this lawyer "in good standing" cannot reasonably deny that he signed the receipt form as requested by the USPS when it (the USPS) delivered my envelope to him.
When you make an assertion in a civil case, you have the onus of proving it on the balance of probabilities with evidence.
A signature on a document is very good evidence that the person whose signature it is has read and understood the document. To prove that they didn't is almost impossible - they would have to prove some sort of unlawful coercement or legal incapactity.
Now, if you say that the signature is forged - you have to prove that assertion.
