Metal ions have access to many electronic states and orbital orientations. They are more flexible in that sense than regular protein atoms. That is why they can help reactions that are very difficult without them. Also outside the context of proteins, metals are good catalysts.

There are may ways in which metal ions are involved in catalysis by enzymes. This is  a very broad topic. Here are a few examples.

In metalloenzymes (e.g. metalloproteases), tightly bound metal ions typically form part of the active site, bind to the substrate, and may have an active function in catalysis.

In many metal-using enzymes, such as ATPases and DNA polymerases, magnesium ions are co-substrates, forming a ternary (3-part) complex with the enzyme and phosphate groups of the substrate.

In some enzymes, such as cytochrome P450s, oxygen bound to an iron ion incorporated into a heme prosthetic group is involved in the electron transfer reactions.

Here is a Wiki page on the subject for some more examples and details:

https://en.wikibooks.org/wiki/Structural_Biochemistry/Enzyme/Metal_Ion_Catalysis

For detailed descriptions of the role of ions in catalysis by individual enzymes, you should delve into the relevant literature.

Metal ions have access to many electronic states and orbital orientations. They are more flexible in that sense than regular protein atoms. That is why they can help reactions that are very difficult without them. Also outside the context of proteins, metals are good catalysts.

Besides the reasons mentioned above by the two previous responders, another is the fact that ions, or electrolytes, affect the ionic strength (I.S.) of the solution in which the protein exists, and I.S. in turn affects the proper folding of proteins, which of course include enzymes.  With inaccurate folding in 3D, enzymes, especially, cannot form the specific 3D microarchitecture required of the active site. This will of course affect the binding or the catalytic activity, or both, of the enzyme.  Hope this makes sense.